Cheers for the help so far - there must have been an issue with the UNRAID vmdk i'd made. Created it again from scratch, uploaded it to the ESXI box, and this time it worked a treat. I've started pre-clearing the disks (3x2tb) and so far so good, except for the fact that the SMART Reports dont seem to be working when using the ./preclear_disk.sh /dev/sdX command. I get the following back:
Should this be happening? All the disks are WD Green drives (WD20EARX) And show as passing the Preclear, but I cant see any of the usual stats.