yes - a few betty swollocks moments but Chris was a star
original disks 0, 1 & 4 all work fine
replacement disks 2 & 3 were needed
As I said above, you can switch the engine off between disks and it picks up where it left off - same when the disk failed halfway through - on putting the new ones in it just carried on from the point of failure and not back to the start
The original disk 3 must have failed right at the end, because once the new one was in, it finished in 4 minutes - disk 4 then took another 4 minutes.
so with Chris' permission, I'll bundle up the "working" set to the first person to PM me and post back here