Certify Error while using the computer?

Hi all,

I have a thunderbay 4 with 4x3TB (which are from an older NAS with Hardware RAID I used to have). And i have a full licence of SOFTRAID 5.5.5 with it. One of those drives failed the certify process so i bought a brand new replacement. This one also failed to certify, twice. So i got the drive replaced again.

Guess what, that new new drive failed the certify process as well, but this time with a weird error halfway through pass 3:

Jan 26 15:20:03 - SoftRAID Application: The certify disk command for disk disk1, SN: WD-WCC4N2NLDTD6, SATA bus 0, id 3 (Thunderbolt) encountered a verify error (offset 797,044,831,232, i/o block size = 16,777,216). Error during random access testing. This disk should be replaced immediately.

So my question is:

could this be caused by the fact that i have been using the machine while the certify process was running? Or maybe because the New Mac Pro has been running for days on end?
And is it worth me trying to re-certify that drive? As in, if it failed once, does it mean it'll fail every time? I'd rather not waste anymore time with this and replace that drive again.
lastly, could it be sign of a hardware failure on that specific slot within the enclosure?

Any help would be much appreciated.
Romain.