Boot up with the disk that has the data you want to keep (import, etc.).  Insert the other disk - it will probably show as foreign - you must CLEAR that foreign configuration, then assign it as a hot-spare ... the rebuild will begin shortly.  NEVER force online a drive in this kind of situation ... had it let you, you would have likely had to rebuild from backup or scratch.
We have a Dell PowerEdge 2950 running with RAID1, two physical disks (I will call them PD0 and PD1 below).  For testing purpose, we pulled PD1, boot up Windows with PD0 only, PASSED.   Then we pulled PD0, plug back PD1, try to boot up Windows with PD1 only. In Virtual Disk Management menu, it can't recognize the RAID array, I need to choose to import "Foreign Config" from PD1. Then Windows boot up, PASSED.   Here comes the problem, I don't know how to make it run with two physical disks again. I plugged back PD0, but it didn't synchronize data from PD1 to PD0. In Virtual Disk Management menu, it said the RAID1 array is degraded, with one physical disk missing. But in Physical Disk Management menu, it did show both two disks exist. I tried to choose "rebuild", "force online", "force offline" on both two disks, all failed with an error message, simply saying it failed. I dare not try other options, afraid that I will lose all data on both disks........  [IMAGE]  
Can't add a physical disk back to the RAID1 array