You need to clean this drive and let the S/W RAID rebuild the array again.
After you plugged the drive back to your DSS, go to webgui -> Setup -> S/W RAID -> MD0 and in the "Manager" frame set the following flags to it:
F - set faulty
R - hot remove
then press "apply".
After that go to webgui -> Setup -> S/W RAID -> "Create new S/W RAID unit" select this hard drive and add it to your S/W RAID as "spare for "MDx". Now let the S/W RAID rebuild the array.

BR,
Tomek