9207-8i is a very good option and compatible with Unraid, though thee could always be some compatibility issue with your particular board or a problem with the HBA itself.
If the disk was unmountable there would be corruption, since it mounted correctly should be fine, but yes, you need to be careful before rebuilding on top, sometimes when this happens, when you know the disk wasn't the problem, it's better to do new config and re-sync/check parity instead or rebuilding the disk on top of the old one.