I have been plagued by this problem for months; when copying large (2-15G files from one drive to another, I find that the source and destination do not match afterward when I run a binary comparison between them. Originally I was transferring files between two XP machines over a gigabit network. I attempted all possible combinations of drive mapping with no effect. Occasionally a recopy *would* match, but often I got no improvement. I am now transferring the files directly on the target machine, by attaching the source hard drive to a spare SATA connection, with the same results. But if I come back to it a day or so later, it may suddenly copy and compare just fine. The original configuration was XP-SP2 to XP-MCE-SP2. I'm now only on the MCE machine, though I have installed SP3 as well as all current critical updates. It is an Intel Core-2 Quad Q6600 with 4GB of RAM. I am running AVG v9 and ZoneAlarm v7, and am using Beyond Compare 3 to verify the file transfers. Sometimes the comparison fails almost immediately, other times near the end of the file. I've also run Spybot and several other anti-malware programs with no hits, as well as memtest86 with no failures. This is a relatively young machine; it was built a couple years ago but gets fairly little use, mainly for bulk storage. If anyone has any ideas or knows how I can fix this problem I would be most grateful. It's been a constant annoyance and I can't find anything that would explain it. At this point I'm out of ideas, other than junking it and starting from scratch with new components. Any help would be greatly appreciated. Thanks!