In OMSA, go to Storage, PERC, Information/Configuration (link at top of page), then Foreign/Clear from dropdown menu of available tasks for the controller.  Then go to Storage, PERC, Virtual Disks, and select Assign Hot-Spare from dropdown menu ... the only drive it should show you is the new one you just put in ... select it to assign it as a hot-spare, then the rebuild should begin shortly.
i have dell 2950 whose having two vds disk 0 & 1 RAID 1 and disk 2,3,4,5 Raid array. i checked that disk 4 shows amber light . Failed  ok now i replaced it with another drive but it shows me as"foreign disk" someone told me you have to clear this  any steps to removed this ? i have dell perc5i controller and also i am worried which disk is my hot spare disk OMSA unable to show me which disk is hot spare ? 
"foreign disk" - How to remove this error