How many hard drives can you have in one computer

Can you just have two or can you put several hard drives in?

It depends on what type of drives and what your motherboard will support. The most common configuration on motherboards allows for up to 4 IDE devices, which can be divided up between hard drives and CD/DVD drives. Some motherboards are configured for RAID arrays which can have multiple hard drives configured for redundancy and large data arrays. These are typically used in servers. Now some hard drives are being sold with a Serial ATA (SATA) inteface, which require a compatible motherboard. There are also SCSI drives which need a special controller.

So the main answer to your question is… “It depends”.

This depends on a lot of factors. One of those factors is what type of drive interface you’re dealing with.

If you’re using EIDE drives, and you don’t have any CD-ROMs connected, you can fit up to four drives on the two EIDE channels that exist on most motherboards. Should you wish to go beyond four drives, you can get a PCI expansion card that has 2 further EIDE channels on it, and then have a maximum of 8 drives. You can continue to do this until your hardware runs out of IRQs.

With SCSI devices, it depends on which type of SCSI you are using. SCSI and Fast SCSI have a maximum of 8 daisy chained devices per SCSI card, as does Ultra2 SCSI. Ultra SCSI and Ultra Wide SCSI go anywhere from 5 - 8 devices maximum per daisy chain, and Ultra2 Wide SCSI, Ultra3 SCSI and Ultra 320 SCSI will support up to 16 devices per daisy chain.

For your Serial ATA hard drives, you will be limited by the number of SATA channels on your motherboard, which may be two or four channels available for connection at any one time. I am not aware of any PCI cards that supply further SATA channels for connection, but they may exist.

Again, any devices requiring IRQs and I/O address ranges will have to have them assigned, such as the Promise card which supplies additional EIDE connections and plugs into a PCI slot, so you will be limited by your available number of IRQs on how many of those you can install. IRQs are typically allocated for the primary and secondary hard drive when using EIDE, and you can use the IRQs that would be allocated for CD-ROM drives for additional EIDE hard drives. The SCSI card uses one IRQ, and each device attached uses a SCSI ID. I am not sure if you can install further SCSI cards on different IRQs or not, but if so you would be limited to the number of available IRQs. There are 15 total IRQs, and any not in use by a device on your computer are available. So for example, if you had 13 of them in use, you could use two further IRQs on which to add PCI EIDE controller cards and have a total of 12 EIDE hard disks in your computer.

Hope this helps.

Going into rather ludicrious land, you can attach HD’s off the USB bus and the firewire bus. Each USB bus can handle up to 127 devices and I think firewire handles 63. A standard motherboard can have 3 USB busses and a firewire bus and 6 more can be added via addon cards making a total of 1206 + the 4 from IDE to make 1210.

A couple of corrections there, in no particular order.

Windows 98 and later as well as Linux let you share some IRQs between PCI cards.

There are only 14 IRQs on a PC. IRQs 2 and 9 are the same.

SCSI Ultra Wide allows for 15, not 16 devices. The 16’th device is the SCSI card.

SCSI allows for 7, not 8 for the same reason.

You can normally have up to 4 hard drives, if you have no other ATA devices (ATA CD-ROM, DVD, ZIP). You could have a SCSI CD-ROM instead for instance. That would allow you to use all of you ATA channels for hard drives.

The Windows ATA driver (at least for WIndows 98 and older) will only support 2 ATA/IDE controllers. If you want to add another one, it will have to have a separate driver. Linux of course doesn’t care, and will allow you as many as you have room for, but you may have BIOS/boot problems with more than two.

It would probably be an EIDE or Serial ATA internal hard drive.

So all I have to do if its an EIDE hard drive is get a PCI card (I have about 3 free slots) that has connectors for an EIDE on it? will that have any effect on how fast the connections work (will it be a bottleneck)? I already have 2 hard drives and a CD-Rom drive, maybe I can fit a 3rd harddrive onto my motherboard if I have 4 connections

As long as the two devices are not going to be using the same IRQ at the same time.

No, there are 15 IRQs, and they are actually numbered from 1 through 15. IRQ 2 still exists, from back in the days when IRQs only went up to 8. Currently IRQ 2 is the cascade IRQ which is the IRQ through which all the IRQs from 9 through 15. The number still exists, and there are technically 15 IRQs, although some of them are dedicated to specific purposes which cannot be changed like IRQ 2.

It’s still 16 devices, although one of the devices is always the SCSI card itself. Next I suppose you’re going to tell me that I forgot to mention that the SCSI cable has to be terminated?

Really? I have a computer sitting on my desk at the moment with 6 EIDE ATA100 hard drives in it, and it currently has both a CD-ROM and a CD-RW on EIDE also. It is possible to add additional channels using something called a Promise card.

It’s possible to install more than 4, but as I already said, it requires additional hardware in the form of a PCI card such as a Promise card.

The above mentioned computer is currently running Windows 2000 and has absolutely no problems. I installed the driver for the PCI based card, plugged in the extra HDs, and away I went.

You’ll have two EIDE connectors on your motherboard, both of which can use a cable that has 3 connectors on it (one end for the motherboard and then there will be one in the middle and one at the other end for drives), not four connectors on the board.

You could potentially plug your new hard drive into the one unused connector on the CD-ROM’s EIDE cable, but you must configure your settings properly and you may experience problems because the drives do not operate at the same speed. The HD will most likely not like to be the slave, which means the CD-ROM would have to be. It’s not a recommended solution, but does sometimes work.

Well, as these guys have said, it’s a lot more than two. In my current system I’ve got 3 HDDs (20GB, 80GB, 200GB) and 3 optical drives, which leaves me 2 drives free on my motherboard (I have 4 IDE controllers on board…two are ATA-RAID, but can be used in non-RAID Ultra ATA 133). Plus, I have two free slots, which I could likely add controller cards to, bringing 8 additional devices on board for a total of 13 hard drives. Plus, if I got rid of my optical drives I could have 16 ATA drives.

This is true with the original PIC model, but all modern systems (pretty much everything past the Pentium 3) use APIC (Advanced Programmable Interrupt Controller) technology that supports a minimum of 24 IRQs. Some APICs support up to 64 IRQs, and it’s possible to daisy chain up to 8 APICs to support up to 512 total IRQs. Since ACPI (Advanced Configuration and Power Interface) allows practically limitless IRQ sharing, IRQ resources are no longer a constraining factor in systems with modern ACPI-compliant hardware.

This Microsoft article has more information about APIC and IRQ assignment on modern systems.

In addition to the fine points above, there is another limiting factor: The number of power connectors in your computer case. I’ve run across that limit before! If your computer is a “build-it-yourself” you are faced with the dilemma of getting rid of some internal I/O device or going out and buying a new case. The one time it happened to me, I had an old tape-drive unit that was doing nothing but take up space since the tapes had long since become woefully inadequate and took it out and used its power connector.

And if your computer is a “brand name”, then the answer is usually “too bad, you’ve reached your limit!”.

You can always buy little Y-splitters that allow you to plug multiple devices into the same power cable. This won’t solve the problem of overall power consumption exceeding the capacity of your power supply, but if it’s just a plug issue, Y-splitters work great.

You’re absolutely right. I just didn’t know the age of the system, and so assumed that it could potentially be one with a maximum of 14 useable IRQs.

Yeah, once you go over wattage you need a bigger power supply…which would then give you more capability.

You can use some interesting combinations to bump up the drive numbers. One of the machines on my desk has a VP6 motherboard, which has controllers for 8 total IDE devices. Also in the case are three IBM server multi-UWSCSI controllers, each of which has three 15-device ports for a total of 45 drives/card * 3 cards = 135 devices + 8 ide + any USB drives you wish to add. The first thing you learn is to put the big drives in external cases with their own power supplies and fans (SCSI runs very hot). The next thing you learn is NOT to bid on a huge lot of 4GB drives, you may just win them.

A tip of the hat to Alan Sheets for pointing out this thread. If anyone wants to buy one of the IBM cards for $1200 drop me an email. I’ll even kick in a couple of 4GB drives :slight_smile:

I think you mean a new power supply. Cases often come with power supplies installed, but they’re separate components.

Power will really be the limiting factor for internal devices. Since I don’t think you can “daisy chain” ATX power supplies, you’re done when you run out of 5V connectors (even with y splitters you’ll start having computer problems).

Next step is to go external. Like others have mentioned USB 2.0 and Firewire both offer external HDs (I have one of each). USB 2.0 does have a limited “power supply” off the bus but that can be fixed using self powered USB HDs.

Older (newer?) Abit boards came with the promise IDE card built in. I had one and used to have all 8 channels in use.

Also remember any drive that you’re going to access (on a PC system running an MS OS) had to have a drive letter. A and B are reserved for floppies in the BIOS so that leaves a max of 24 drive letters for accessable volumes.

Not sure if this limitation is removed with other OS’s

Ah yes, and once you run out of volume letters, you can have the wonderfu fun of spanning volumes across multiple drives. :slight_smile:

You can install multiple power supplies in a single computer. You just have to put a jumper across two of the wires on the motherboard connector that is not hooked up.

Most other operating systems have no concept of drive letters, so I don’t think that’s a limitation.

It might not even be a limitation on Windows 2000/XP, since you can mount drives with a path instead of a letter.

I was lurking the boards and came upon this thread. Since Guy and I are instructors at a technical college, we decided to pose this question to our students for consideration.

My answer is as follows. I’m assuming that the starting condition is going to be one of the nice P4/2000 ATX-form factor boards we have in our programming lab (your choice of equipment may vary, of course). I’ve tested each of the recommendations I make singly, and in theory there should be no resource conflicts. However, when you throw all of this together there may be hidden issues.

A System V Unix is my OS of choice here. I’m also assuming that I’m providing enough external power to run everything.

onboard floppy controllers 5 drives (4 floppy, 1 QIC tape)

onboard IDE Primary 2 drives
onboard IDE Secondary 2 drives

onboard IDE Raid Pri 2 Drives
onboard IDE Raid sec 2 Drives

onboard UW/SCSI3
31 ID’s
4 LUNs 124 drives

onboard SATA 4 drives

3 Onboard USB Busses
127 ids per bus
1 UW/SCSI3 coverter per id
31 Id’s per converter
4 luns per ID 47244 drives
onboard firewire bus
63 ids
1 UW/SCSI3 coverter per id
31 Id’s per converter
4 luns per ID 7812 drives

RAMDrive 23 drives

6 pci USB busses
127 ids per bus
1 UW/SCSI3 coverter per id
31 Id’s per converter
4 luns per ID 94488 drives

1 onboard parallel port
1 SCSI1 converter per port
7 id’s per converter
4 luns per id 28 drives

2 onboard serial ports 2 drives

Total number of physical (non-partitioned) devices 149738 drives

Some notes here:

  1. The processor is going to running all-out trying to manage even a fraction of these resources.
  2. If I disable as much external IO as possible (keyboards, mice, and whatnot), I won’t have enough IRQs to run everything. For best results, you’ll have to turn off the floppy controller, the serial UARTs, the parallel UARTS, the sound card, the NIC, the keyboard UART, and the mouse bus. Of course, that eliminates some of the drives I’m mentioned, but that does free up some resources for the PCI USB cards.
  3. Each USB bus, as I’ve mentioned, takes an IRQ. Each usb-to-scsi converter gets a virtual IRQ under the real IRQ. I’ve run 2 or 3 converters (with 1 device each) on a usb bus before, but my proposal goes way over the top.
  4. I don’t have the cite handy, but the highest number of devices that I’ve heard about successfully running on a USB bus is only 119 (including hubs, btw).
  5. Unless there is on the market a 127 port USB hub (active and powered, of course), then you’ll have to sacrifice drives to get powered hubs on the bus. Same issue with the FW port, btw.
    Comments, complaints, flames, additions, refinements, etc., are welcome.