Then your employer could just refuse to allow you to remove the drive from their premises, since you'd have no way of proving that it doesn't contain the employer's data. Which may not be so bad, if you have another copy of the backup somewhere. The worst-case scenario would be that your primary backup drive fails on the day you get fired.

The IT staff wiping the drive to their satisfaction should obviate that concern, I'd imagine.

And the worst-case scenario would be that happening, and your primary backup drive failing, and your running drive failing, on the day you get fired. A failed primary backup drive just means you're out a backup; it takes a failure of the drive you're backing up for the situation to achieve disastrous proportions.