Although it's possible that you need the controller to correctly see the mirrored data on the drive, I would guess that either drive should be mountable by itself too. I'm no RAID expert, but I'm reasonably sure that this is true at least for software RAID 1.
The thing that makes me suspicious that something else is wrong is the 'device does not exist' error you mentioned.
The first thing is to be absolutely sure that your kernel is seeing the hard drive, and know what its device name is. Are you sure it's sda? You can either read the output of
to figure it out, or post the output here and I'll try help out.
Secondly, what exactly are you typing to try mount the drive?