If you were using 2 disks together in Raid0 configuration, the data is striped across both drive items. Removal of one of the drives would result in most of the data lost (except for that which fits under the stripe size)
The data from the 2 drives should be transfered out to a backup, while still configured the same way. Then the drive should be properly removed from the raid configuation in the raid bios, or software. Once that is completed the drive should be reformatted , and then could easily be used as a single drive.
You can leave most of these bioses set for raid (and still have one "spare" on there) or switch to AHCI, it is not known well that most of the raid settings were Raid&ACHI. My understanding of this is low because I am always set for Raid.
If the raid configuration was not raid0 then little of the above would apply.
Sometimes you have to be very carefull to make sure you remove the raid designations, like on most hardware, before returning a drive to single drive service, just depends.