PW protect an external HD? StorageCrypt?

What's the best way to retroactively apply pass-word protection, for an external hard
drive (USB 2) that is already loaded?

I did some searching, and found a package called Storage Crypt, shareware
(3.01 version), for $20. Anyone have any experience with it?

Any other suggestions?

You should check whether Storagecrypt supports on-the-fly disk encryption. If necessary drop them a mail and ask. I expect it doesn't. If i'm right you need to move the data elsewhere before starting to encrypt and move your data back afterward. You can then also save 20 bucks and go with Truecrypt which is free, supports stronger encryption and has some nice additional features.