There are two harddrives listed in ‘My Computer’. I only have one. It is not listed anywhere else, not even in the device manager.
Your harddrive is probably partitioned. If the size of the 2 drives you see in my computer add up to about the size on the label of your hard drive this is probably the case. You could make the partitions into one, but it would destroy everything on the drive.
Not knowing much about what operating system you have; I will take a stab.
On modern Microsoft OSs it will see each partition as a virtual drive.
So, even though you have just one physical hard drive, if it is partitioned as two drives, it will show each partition as a separate drive.
If you did not know that you had an additional partition, post some more info and I will see if that space can be freed up so you can use it.
What Operating System do you have?
How old is you machine?
What kind is it?
If you have a Mac, forget everything I just said. 
Partition Magic can merge any and all partitions and keep all the data intact in “transfer folders”.
No, it is not a partition problem. The computer literally thinks that there is a separate drive. When I double click on its icon in ‘My Computer’ I am asked to insert a disk. Other programs recognize this phantom drive too, Norton will inform me that it cannot access hard drive M when trying to do a system tune-up.
If it is asking you to insert a disk, it must think you have a removable unit. Was a ZIP or JAZ drive (internal or external) ever installed on this machine?
Right-click (we are talking PC, aren’t we?) on the drive icon in My Computer & select properties. Does that tell you anything?
No, it thinks that it is a hard drive. Yes, a PC with Win XP
You could be looking at a network drive or a subst ? drive which is a folder displayed as a drive letter. Look in autoexec.bat and see if you have anything like:
subst d: c:\windows
FWIW your description of the existing drive problem could well be a phantom partiton issue ie “The computer literally thinks that there is a separate drive. When I double click on its icon in ‘My Computer’ I am asked to insert a disk. Other programs recognize this phantom drive too, Norton will inform me that it cannot access hard drive M when trying to do a system tune-up.”
To determine definitively if it is a partition or a sticky BIOS ID issue go into your BIOS and have it do an auto-recognize/ID sequence for the existing hard drives. The existing mechanical (not virtual) drives will be identified and listed. Then save these changes and exist. If a second hard drive is not identified by the BIOS and if the drive is still there (to XP) after these procedures it was not a BIOS ID or partiton issue.
BTW Drive “M” is usally a drive letter that manufacturers will assign to a CD or plug in drive (ZIP, memory card reader etc) not a hard drive. If the drive is being recognized as drive “M” I doubt the system assigned it there automatically as it is well down the drive chain in letter assignment.
Sorry mis-type -“If a second hard drive is not identified by the BIOS and if the drive is still there (to XP) after these procedures it was not a BIOS ID issue but may well be a partiton issue.”
Maybe you set the second HD in the bios to something? If so, change it to ‘not installed’…
To see if it is a Windows problem, go into BIOS and have it auto recognise all the drives. If two drives shows up there, it’s a BIOS issue.
If you have a compressed drive, assignments show up like that & you can’t write to it either.