It appears that I have a failed drive in my RAID10 it consists of 4 drives, and I was in the process of copying the contents to a new RAID10 I've build when the old raid appears to have ground to a halt.
Issuing a cat /proc/mdadm gives
Code
Personalities : [raid10]
md127 : active raid10 sdg[0] sdj[3] sdi[2] sdh[1]
15627790336 blocks super 1.2 512K chunks 2 near-copies [4/4] [UUUU]
md0 : inactive sda[0] sdb[5](S) sdd[4](S) sdc[1]
15628070240 blocks super 1.2
unused devices: <none>
which shows that 2 of the drives have been loaded as spare, so then I decided to check which drives were uptodate, with mdadm -E /dev/sd[abcd] | egrep 'Event|/dev'
Code
/dev/sda:
Events : 5839245
/dev/sdb:
Events : 5839245
/dev/sdc:
Events : 5839245
/dev/sdd:
Events : 0
To me this looks like sdd is failed, but I can't understand why sdb is also been marked as spare, I do have a spare disk that I can add into the array, can anybody advise how best to rebuild the array??
(I have the spare in a USB3 enclosure so that I can add it without swapping cabling etc if required)
Thanks Dave