MemLocker is a safe, fast and easy to use solution for Pocket PC for protecting information on external storage cards. It locks inserted cards when you're switching off your Pocket PC and automatically unlock them when you switch it on and enter the password for your device.
It seems this is the first soft that controls the access to SD card on PDA. In contrast to encryption software it blocks card entirely and do it automatically and very fast. I've tested it with my Atom Exec O2 with Windows Mobile 5. It works perfectly.
Unfortunately it works only on PDAs with Windows Mobile.