WHAT NOT TO DO IF YOU SUSPECT A FAILING DRIVE: Use Chkdsk
*snort* Or open up the drive on the bench, and try to manually fix the head, then run it open... Or try to move the platters to another drive (Not even same manufacturer). Or 'freeze' it. Or 'tap' it.
I kid you not, I've seen it all.
Check drive SMART. If no common signs of failing drive and SMART comes back 100%, Short Test. If any signs of issues, contact client. I vary my procedure slightly from other people at this point. I specifically go straight for the client's critical data first. If that is successful with no errors, I do a compressed intelligent clone and confirm the clone is in good shape, then I do 2 full tests on the drive to ensure there is no issues. I do still warn the client and encourage them into a new drive.
I actually have an entire flow I follow that is far more complex than above. If at ANY point I feel there is sufficient risk to recovering data, I halt and then prepare to refer the client and drive to a specialist.
I really should get it down on paper sometime. My logic tree is pretty wide, taking into effect my experiences as well as ones found here.
Who all tests hard drives every time they come in, first thing after getting a clean boot? <hands up> - First thing I always do. Get system booted (if it's not a HD issue already) or yank the drive depending on complexity of getting into the system, and check the drive first. Comes in for quick help with email? Check drive. Comes in for a bad power jack? Check drive. Virus? Check drive.