The following info may be common knowledge to some, but I got badly burned by not knowing it.

There are two versions of WD "My Passport Essential" Portable Hard Drives. They are entirely different products.

The earlier model (eg. WD3200ME-01) contains WD Sync software which synchronises files to other computers.
If you use WD Sync, thinking you are doing backup, you'll end up with hundreds of inaccessible encripted files, which are deleted if the original files are deleted. WD Sync is not a backup program.
This model has been abandoned by WD, and does not feature on their website.

The later model (eg. WDBAAA3200ABK), which is dimensionally slightly smaller than the earlier model - comes with WD SmartDrive software, which is purely for local backup.

My local computer shop only displays and sells the earlier version - Which is how I came to get burned.