Thanks Chris, I did that and when it restarted two drives, U:\(Bay7) and V:\(Bay8) were the only 2 not automatically back in the pool. U:\ added fine, but V:\ won't add still (same error).
V:\ contains two folders: PoolPart.0e0bba01-fb9a-485c-9ea7-e6923e697750 and PoolPart.2a171a06-779d-4d50-ac16-dcab65358701, U:\ contains one folder PoolPart.81e0a870-a269-4ae9-a4e0-9d3be96a8444, I only have one other drive with two folders, W:\(Bay9) and these are named the same as those in V:\ which seems to have been the error. They both had a tiny amount of files in one, loads in the other so have copied the data to the larger folder in each, then moved the Pool folder and the .coverfs folder in there to a new Old folder on the root of the drive. This has let me re-add the drive. I think I had lost one of them on a re-boot and the software has somehow mixed the two and created new folders for the other drive on each one. Looks right now but will see if it runs right later on tonight.