What has your interaction been like to date? Requested 3 times over the phone or...? Knowing some of these hard drive shops, chances are this is an IT shop first and a business second. They may just be badly unorganized. I can't imagine any legit data recovery shop lasting long with some weird hostage thing going on as regular business.

If it were me, I'd probably try the easy way one more time - call them 24 hours in advance and tell them you will be at their location the following day to pick up the drive. That way if they've misplaced it, they have the day to search for it and have it ready for you. Bring a witness, but keep it professional and not threatening - I wouldn't probably bother with a lawyer.

If needed, you could send a registered letter on an attorney's letterhead. If it's to that point though, you're likely to have other issues, and you'll need to consult whatever paperwork you signed when you dropped off the drive to see what you may have agreed to.

I still go back to it's really unlikely that they're actually refusing to return your drive maliciously. Bad sales guy maybe. Maybe they lost/broke your drive and are trying to figure out what to do. Most likely scenario is they just aren't on top of their own business because they are techies. Don't get angry, just be firm and tell them you'll be there the next day to pick it up and to please have it ready.

Good luck, and use a backup next time (says the guy who has been in the same boat several times).