We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
You're right to be concerned, so many stories out there of techs discovering data on phones, laptops, hard drives, etc.
If the drive is still working, fully wipe the data, there's a variety of shredders and secure delete options out there, depending on what platform you're running it with.
It's a good idea to keep your important data where privacy matters separate from all the other stuff you collect. E.g. keep your documents and private photos and their backups on separate (smaller, cheaper to replace) devices. In worst case you can sacrifice.
Keep the less important data that needs all the space (erm. those Linux ISOs) on the larger expensive disks. If you send those in, in case a replacement is required, then it probably won't matter too much if someone must snoop over it (unless it's too much of the dirty Linux ISOs) :)
Aka have clear device-level separations between any sort of media collections and your critical data.
Zero or scramble it or don't send it.
If you had the drive encrypted to begin with, then it doesn't really matter.
Has anyone ever heard of something bad coming from techs at a hard drive company that went through the contents of a returned drive and then took malicious actions? If they fix a drive and decide to resell it, do they zero out or reformat the drive? (Assuming you deleted the data from the drive and deleted the partition but didn’t do the whole “spend hours zeroing out the drive”)