Ok, so this is really starting to annoy me now. Disk 4 has dropped again and was reporting unmountable, so stopped array, rebooted, disk was there, start array again and it's rebuilding the disk. No idea why. This is the brand new disk I'd previously reported I was using. It's supposedly successfully rebuilt into the array several times now, and just keeps dropping off and being forgotten.
This disk is also using the SAS -> SATA breakout cable again which I suspect may be faulty. Worthwhile replacing cable or is there anything else this could be?
Cheers,
-Chris
tower-diagnostics-20190609-1502.zip