Hmmm...I may have stumbled on the answer. It appears that the "new" disk was being viewed as though it had a foreign (RAID) configuration on it. I was able to "clear" that foreign configuration, and the disk appeared as "ready". I then added it as a hot spare, and a few moments later, it's status changed to "rebuilding". Fingers crossed...
I'm having a number of issues in replacing a failed drive in my server. My configuration is a little "oddball", in that I have two RAID controllers in the box - an H310 connected to two SATA drives in the 5.25" ODD bays (RAID1/OS), and an H710P connected to the backplane, running a RAID6 array (Data).   Recently, one of the disks in the RAID1 array was marked as "failed", and the array marked as "degraded". I removed the failed disk, and replaced with a near-identical unit (same size, type etc.) That replacement disk is now marked as "foreign", and I'm unable to find any way to add it to the array. Whether I use the H310 BIOS configuration utility or OMSA, it is always marked as "foreign", and I'm unable to utilise it in the array. I am unable to add it as a hot spare, or indeed do anything else with it.   I'm beginning to think that I'll need to pull the original (still good) disk, and replace it with one identical to the "new" replacement I installed and completely reinstall the OS (which I'm obviously keen not to do).   Is there any way I can "force" the controller to add the new disk to that RAID1 array, and thus have the array rebuilt?[IMAGE]
T320/H310 Failed drive replacement in RAID1 array - replacement disk marked as 
foreign.