I've been using PGP's Whole Disk Encryption for Mac, and it generally works well. However, I discovered that if you encrypt say, a USB flash stick, and then decrypt it, it no longer will mount properly on a Windows XP or Vista system. The disk continues to work on Macs, and I even was able to use it on a Pioneer DVD player with USB mode, but inserting it into a Windows machine results in a prompt to format before use. Disk Utility.app on the Mac shows only 1 partition, but Disk Management on Windows XP/Vista shows 2 partitions: a 200MB protected partition (can't delete it), and the remaining space in a second data partition. When I tried to mount the second partition in Disk Management, it says that it will work only after a restart (but not surprisingly, a restart has no effect).
So here's how to fix it, assuming you only have access to Windoze machines and Macs. You will lose all your data on the drive, so back it up.
1. Attach drive to Windows machine. Format when prompted. It will format the protected 200GB partition, but you will still not be able to delete it in Disk Management.
2. Attach drive to Mac and run Disk Utility.app. Now that the protected partition has been formatted, you will see 2 partitions. Re- partition the drive so it only has 1 partition, and it will now work properly on both Macs and Windows machines.
The fact that PGP leaves a hidden, protected partition is infuriating, but is easy enough to resolve using the above steps.