Absolutely do not tell them. This is a can of worms you really do not want to open. You will not be met with a welcoming response. Its sad, but that is the state of affairs.
What can of worms exactly is opened with a ‘Hey I bought this drive on eBay that seems to have your info on it - want it back?’
What plausible damage have you done that they could sue you over? If all you want is your cost or the like, I can’t imagine that is a crime either.
If someone has a lot to lose, I guess sending a letter through a law office would be a sane option, but backblaze doesn’t strike me as the shoot-first-ask-questions-in-the-deposition type of company anyway.
I'd tell them anonymously. Register a new protonmail account and email support at Backblaze with some incriminating evidence that you're actually in possession of what you claim, and then offer to mail the drive back to them.
What plausible damage have you done that they could sue you over? If all you want is your cost or the like, I can’t imagine that is a crime either.
If someone has a lot to lose, I guess sending a letter through a law office would be a sane option, but backblaze doesn’t strike me as the shoot-first-ask-questions-in-the-deposition type of company anyway.