disk1 of raid1 installed on new main board with same chipset.
Was recognized as part of raid1. Added new spare drive and is rebuilding now.
If I had placed the 2 original disks, maybe they would have worked also without rebuilding. But I couldn't risk losing data so I kept one disk sagely away.
It is always good to back up all your information in case of any failure, so in this case if you want to boot your system with only one drive that will be ok but since you have the same chipset with the replacement board I don’t think you will have any problem.
If you already boot with one hard drive go ahead and back up your information and then you can go ahead and add the second hard drive, you can do it either way and I’m sure you will not have any problems.
If your replacement board has different chipset then you will need to reinstall the operating system and start from scratch.