The Bat! Private Disk

From RitLabs:

The Bat! Private Disk is an easy-to-use, reliable, user-friendly and smart program that lets you create encrypted disk partitions (drive letters) to keep your private and confidential data secure.
The Bat! Private Disk prolongs the life of a flash-disk and improves the write speed to the media by caching write operations to minimize the number of actual writes to the chip. When your application program writes data to an encrypted disk partition, Private Disk delays the actual write by keeping the data in memory, storing the data until later or for when you un-mount the encrypted partition.
The encrypted partition resides in a single file which is easy to backup to any media like a recordable CD or DVD to protect it from data loss in case of hard drive failure. The Bat! Private Disk significantly improves the portability of your data, since it's easier to transfer a single file than a folder hierarchy. The Bat! Private Disk safeguards all your data with no performance penalty.