This is actually fairly common. Change the sata cables on the disks so disk 0 becomes disk 1 and vice versa. Then power on, this forces the card the do a hard rescan. The replacement should be detected. If it doesn't start re-syncing just set it as a hot spare and reboot. It will start rebuild then.
I have a T100 w/ SAS-6/ir RAID 1 2x250gb's and one of the mirror drives is dying (predicted fail), but when I install a replacement larger drive of same type (SATA, 7200 spin/1TB and another 250gb) the drive simply is not seen in the raid bios utilities and the instructions simply say it should be installed and instantly start synching....?  is there something I'm missing?  I've tried a brand new uninitialized disk, an initialized disk, a disk with a exact match partition to the MB of the original mirror and nothing seems to be recognized except the original disk which is marked for predicted fail?  I must be missing something here.  Can someone please give me an assist?  I've loaded Openmanage and can see the RAID and see both the good disk and the degraded disk.  (I never see any of the replacement disks when installed).   Will I have to order an exact model match to get this thing to recognize a new drive?  OS: W2k9 r2 64 bit All the latest firmwares on the RAID.  
SAS-6/ir Raid 1 - degraded, one drive is failing, replacement drives not being 
recognized or synch'd?