Is it possible to mix and match disk types and capacity?
I have built my RAID with 4 x HGST 3TB disks, but one is showing as liable to fail. Would it be OK if I was to replace it with, say, a 6TB disk from a different manufacturer?
Equally importantly, what spec should my drive(s) have for video editing? I know 7200rpm, but what else?
Any drives will do. There are no issues with different/larger drives.
Best advice is be careful to avoid SMR recording drives.
(CMR is correct) SMR are cheaper, and are very slow on sustained writes. They are inferior drives and are now what you generally get when you get inexpensive drives, but some manufacturers are putting SMR throughout their product lines, even within the same model (sometimes even at a higher cost!)
People have tried to make charts of CMR vs SMR drives, but this information changes, so is hard to keep up with. Just be sure when you purchase you can find a spec and it says CMR recording (conventional).
OK brilliant, thank you so much for this info.