Mismatched RAID 1, what can I expect?

I have a RAID1 array, but have never had to fix it. My recollection, though, is that the RAID controller should be able to handle this. It certainly will be much simpler if the replacement unit is LARGER than the original. In that case, the RAID controller should simply create on the replcement a new partition the same size as the old one (matches partition of the existing good half of the array) and use it to rebuild the second copy. You'll be back to a fully functional RAID1. There will be an unused (and unusable) part of the new HDD that is unallocated. But I doubt you would be able to create in that spare space another partition to be used as a "normal" logical disk.

+1 for you both.

The spare capacity on the larger replacement drive won't be available to the raid system, as both drives need to support the same size partition as mirrors.

Performance issues were all about having somebody try to raid a faster rpm drive with a slower rpm drive. The raid will be established, but you're wasting your money on one of them due to speed difference.

You've talked a bit about rpm differences, but what if the two drives are different connectors? I have one SATA hard drive and one IDE hard drive. Could I use both of those for RAID 1?