[lug] hd encryption guidance

[karl horlen]

if i've got an external usb hd that is not encrypted that i want to encrypt, is that possible or must you encrypt the file system before you put any data on it?

the same question would apply to an internal hd.  if i have two drives in my system and one is the system disk (win xp) plus data and the other is pure data, can i encrypt these after the fact?

fyi, win xp based system.

thanks