I's probably dump the drives and start over, since drives are so cheap now. I think you have a 3-year warranty on the drives themselves, right?
Yes, manufacturers have their own test utilities which you can find on the various web sites. Also, there are plenty of utilities that will read the SMART data off a drive so you can see what the problems are. Acronis has a free one, as do many others.
I don't know much about the Netgear stuff, I think there are two possible drive formats neither of which are readable on Windows, so whatever you do you'll be reformatting drives on the PC to do much useful.
If SMART shows no errors, then I guess you can re-use the drives, re-initialising them once again in the NAS. But if one of my drives so much as farts funny, I bin it.