I used a borrowed Mac to transfer sensitive data directly between two USB thumb drives. Ideally, I’d like to not leave a single trace of the files and transfers on the borrowed Mac, but I understand that a modern OS might store a lot of tiny bits of information anywhere and everywhere. What are these on MacOS? (In my case, specifically, MacOS 10.13.6 High Sierra)

I noticed that I couldn’t safely eject the drives because Spotlight were indexing them for a pretty long time. But as far as I have understood, these indexes are only placed on the external drives themselves, and not anywhere on the Mac. Is this assumption correct?

In short:

• which processes might locally store information of externally mounted drives,
• what kind of information are they storing, and
• how can I “clean up” before giving the Mac back to its owner?