You cannot make a drive
private, You can only make a
folder Private and it must be under your User Name under
C:\Documents and Settings\YourUserName (ex) My Documents, My Music etc. Then the
Private check box will be available.
You can
Encrypt folders outside of your user profile name in XP, but not a whole drive, without 3rd party software. You can
encrypt an External drive using Windows 7 and and it will remain encrypted in XP, but you can't change any of the files until you return it to Windows 7.