Why? Couldn't you just format the drives if you were that paranoid? Are they worried about some kind of hardware bugging or sub OS exploits while the owner is not looking?
https://www.defcon.org/images/defcon-19/dc-19-presentations/...
(Note slide 116 there - about 7 up from the end: "Attacking the OS kernel")
https://www.v3.co.uk/v3-uk/news/2099616/black-hat-charlie-mi...
http://www.karosium.com/2016/08/smbusb-hacking-smart-batteri...
Check out: https://www.malwaretech.com/2015/04/hard-disk-firmware-hacki...