Replacing faulted disk .

Hi

I had a pool , that contain 10 (600GB) disks . also had 1 more disk for hot-spare .

in 2 day , 2 disk faulted . now i replaced the disk with new . but the new disk's shown in "Unbound" state .

i used raid 4+1 for my pool , so now i must have 1 unbounded disk and 1 disk must used for raid .

what is the problem ? why i have 2 unbound ?