I had to borrow a friend's Macbook to run Win for a couple of months in 2017 (2 laptops lost in a flood).

I "thought" I wiped the drive properly, but...I saw on my credit card site, a device was listed as "Mac OSX" accessing the site August 2019!

Friend now wants to sell the Macbook, and I'm concerned my info is still on their HDD.

Any ideas on what I did or did not do correctly?