Computer help forum: I/O Device Error and External USB Drives?

I am running Windows XP Pro on a Dell XPS Desktop (PIII). After significant data loss using Netgear's Storage Central due to a bad firmware/software release, I switched to external drives. I am using a 320GB Maxtor One Touch III USB 2.0 External drive, plus two internal drives (C & D). Everything was working fine and all drives were functioning properly, until I tried to add a powered USB hub and another external drive, for an extra back-up. This has given my system the oddest problem... I can copy data (small & large amounts) from the internal drives to the external drives. But when I try to copy or move files from the external drives to the internal drives, I get the following error: "Cannot copy (filename): The request could not be performed because of an I/O device error." I used the device manager to uninstall everything related to the USB ports, including the hub, then reinstalled only the printer and the Maxtor drive on the PC's primary USB ports. Still the same problem.

And just so people don't have to repeat themselves, I have already performed a chkdsk /f on the Maxtor drive. I have also reformatted the drive, and backed up everything again. I was able to see the data on the Maxtor drive and I was able to write all the data I wanted to back-up over to it, I just can't copy it back to the C or D drives. I can get some files to copy back (maybe 1 or 2 files at a time), but essentially I only have a one-way flow of data.

I have tried just about everything I can think of. Now, I need some expert assistance. Thanks.