StealthDisk

From Security Group Six:

StealthDisk provides a transparent way to easily and securely store all your important files and documents. Using industry-leading encryption algorithms, StealthDisk creates an encrypted virtual disk drive that can be used as any regular disk drive. All data is encrypted and decrypted on-the-fly as you work. When you are done, simply dismount the StealthDisk volume and all your important files will remain encrypted. There is no need to encrypt each and every file or each and every every folder as other encryption applications may require.