Do you think that both the USB and Firewire cable are bad? That is fairly unlikely
If you have tried both, chances that both cables are bad is remote.
So that makes me think that it might have something to do with your hardware connection (between the cables and the drive).
Let's see if I have this correct:
- The drive works. You've tried the Repair function with Disk Utilities and it worked fine.
- The computer can find the external drive using either a FW or a USB cable.
- You can transfer small files. But large files will not transfer (write to the external).
- The files that you are attempting to write to the external are not corrupted. You have tried copying them in place on the internal HD with no problems.
The only thing that I can think of at this point is that you have something intermittent in the hardware of the external drive. It could be the connectors, the board, etc.
FWIW, I have one external FW drive this happened to me. It can see the drive but sometimes the drive disappears and I get an error. The connector was loose. Use the 2nd connector now with no problems. I've also had problems with the board/chip set on another external. I have a few external HDs -- about 10 -- so it can happen.
This is why I am thinking that it may be a problem there.
One easy test, is to get/barrow another external HD from a friend and try it out with your test file. The results of this test may help us diagnose your problem.