Before you can introduce a used drive into a RAID array, you need to remove the meta data regarding the previous array from that drive. The scenario is you have an old server with old drives in a RAID5 array, one fails, and you can only acquire refurbished exact replacement drives.
When using an Adaptech 2400 (maybe other) SCSI / RAID controller, you can do this at the BIOS / SMOR level.
Highlight the new refurbished drive, and press ALT-F10-Z. You will mysteriously now have a "Zap drive" option available on the Action menu. Choose to zap it and it can now be used to rebuild the existing degraded array.