How does the true speed of an external hard drive compare to the true speed of an internal hard drive? Is the speed of USB 2.0 or Firewire a limiting factor? Is there another bottleneck somewhere?
The reason I ask is that I can’t bear the slow speed of 5400RPM hard drives. If it weren’t for the advent of the 7200RPM hard drive I would have committed seppuku long ago. I would hate to buy an external 7200RPM drive only to have it perform more like a 5400RPM drive.
I asked a friend about this at lunch and he said that he can’t tell the difference between his internal and external drives, but admitted that he only uses the external drive for backup, so doesn’t have any real numbers for comparison.
I would think that USB 2.0 or Firewire would be a limiting factor in transfer rates, but perhaps I’m misunderestimating the bandwidth 
Unless you’re transmitting targeting data prior to the counterstrike impact of Russian ICBMs, does an extra 45 seconds really shake your world that much? 
Your three big factors are going to be (a) the speed of the hard drive, (b) the speed of the interface, and © any OS/buffering/motherboard bottleneck factors.
You can’t do anything about ©, (a) is dependent on what you’ve got in the case, and so the only remaining factor is (b). My understanding is that, while USB 2.0 is theoretically rated faster than Firewire/IEEE1394, in reality Firewire smokes it – USB 2.0 has too many collisions with ACK/NAKs and other overhead traffic that lowers the effective transfer rate far below the theoretical maximum. Firewire, on the other hand, was designed for guaranteed continuous high-bandwidth delivery (note the higher number of video devices that use Firewire over USB 2.0).
Personally, I’d go with Firewire 800/IEEE 1394b and a fast external hard drive if you can swing it. If not, I’d take vanilla Firewire on a dedicated port over USB 2.0 any day.
Firewire has a bandwidth of 400 mbits/sec. A 7200 RPM drive has a general max transfer rate of 110 mbits/sec.
(cite: http://www.tomshardware.com/storage/200311141/hd-250-07.html)
IIRC USB2 has a 480 mbit transfer rate. You will not notice a difference. I personally have Firewire HD and 3 USB2 drives and I like USB a little more since it’s standard on more mainboards (my board did not come with built-in firewire).
Also, according to many of the benchmarks I’m looking at (all at tomshardware) there is very little difference between a 5400 and 7200 drive (and even a 10,000 rpm barricuda). The actual drive quality is a bigger factor than “large numbers”.