Could well be USB attached. There's some windows setting (or was on XP) about "optimize for quick removal" (vs optimize for performance). The upshot of this was that it flushes the FAT to the drive very often by default on removable drives, which:

a) Slows things down hugely
b) Could result in allocations for files being cut down to actual size part-way through a copy, which means it'd then be likely to fragment when the file copy continued.