Which disk used by system to boot in raid 1

If it's a bootable RAID 1 array, then it's using both drives, however it should be able to run off a single drive in degraded mode unless of course a virus messed up the files on those drives or you accidently deleted a needed system file. Remember, in RAID 1 whatever gets written to one drive gets writen to both. RAID is not a backup solution. It's only meant for redundancy in order to keep a system running if a drive fails.
 

popatim

Titan
Moderator
Typically whichever drive responds that it had the file ready to transfer first is the one that will be used for that one file. Thats how raid 1 is supposed to work but I have seen s few instances of 1 drive always being the primary drive and the other a dedicated mirror that never gets read. I've also seen a few cases where the drives altrernate which one gets to be the primary drive.