Erase the drive. I Guarantee the owner will suddenly call you, looking for it and the files, and some change orders, within 24 hours. It’s like voodoo, every time.
More practical answer; That you have that many drives coming and going must mean you’re doing a lot of business, so congratulations on that. But it sounds like you need a better way to organize and track these physical assets during the intake phase. That’s the most practical place and time to do it, as it comes in the door. Make your own label system if you need to and stick it on the drive somewhere the day it comes in the door, with some code to associate it with the specific client. At least write a client name on it with a grease pencil or sharpie somewhere. Then you always know, and you can mail the drive back with their bill, or whatever. Or at least know whom to contact.
I would not re-use other client’s drives. Or send them out to others with my product on them. I wouldn’t trust them unless I knew more about them. My and my organization’s reputation is attached to the product. After all, you are trying to present your product as well as possible, and what does it say about, say, your hamburger, if it’s wrapped in a re-used wrapper from another restaurant? I don’t stake my reputation on mystery hardware I didn’t acquire. And it somehow just doesn’t seem… polite. They will come looking for it eventually, so I’d put it in a sealed plastic box with some desiccant and label it with a date at least.
Back in the day, when the world was young and puppies were the oldest animal, tape dubs were often made on recycled stock. You were supposed to bulk-erase it and/or”black” the tape by recording black video or bars and silence over it, then you would put your new content on it, scrape off any old labels as necessary, and ship it… saving money. If the tapes were new with just a few passes on them, it was no big deal. But people get sloppy and complacent. They started skipping steps like pre-blacking the tape, figuring the recording of the content itself is good enough to cover over the previous material.
I leave it to your imagination, what can, and sometimes did, happen.
Good and important clients get clean, new drives. Always.