Willie Hecke said:
...sure enough, the 3TB one had a yellow 'Warning' flagged up, with the culprit, as you can see from the screen caps - being the '05 Reallocated Sector Count' . What I'm a bit confused about though, is the other drives, which were blue 'Good' flags, ALL had a higher reading than the one flagged up on this score...but classed as good?
It's the Raw Values you should look at, these are the actual count of reallocated sectors. For the 3TB 'bad' disk there were 0x46 (70 decimal). The good disk has zero. The 3TB's Read error rate looks worrying too, that should also be zero.

I wouldn't trust this disk for archive purposes, it could degrade at any time.