This may also apply to previous versions of Windows, but I haven't checked yet. I'm running Win 7 x64 RTM and have one of my external USB HD's device encrypted with Truecrypt 6.2a I noticed that after restarting Windows, the Truecrypt header was corrupted, although the backup header was OK (and so could be used to restore the main header). At first I couldn't work what the problem was, but eventually noticed that if I switched my USB drive off before rebooting, the Truecrypt header was not corrupted. I compared the entire drive before and after corruption, and found the only difference was that Windows had overwritten a block of 64 bytes with zero, starting at offset 446 (from the beginning of the drive), which happens to correspond with the MBR partition table. Not completely sure yet, but it looks like the disk is being initialized automatically on either shutdown or startup (ie. if a disk is found which has/appears to be a corrupted MBR partition table, then it is automatically initialized to zero). Obviously this is an annoying problem when you have Truecrypt encrypted devices, as you have to remember to turn them off before rebooting, otherwise be prepared to restore the backup header. Anyone know anything about this, and whether it's possible to disable this automatic disk initialization ?