i use folderlock Folder Lock - File Lock and File Encryption Software Free Download some people hate it and have nothing but trouble others love it, ive used different versions over the years and found its easy to use.
i use manly for usb memory sticks but it works well on complete drives to.
like all software encryption it is advisable to have a backup, i am sure any encryption software will baulk if you loose power to drive if its writing...