I have a raid 5 setup through IRST. I want to disassemble the raid, but keep the data. It is currently 4 hard drives. I have enough external space to back up most of the data, but I will need one of the raid 5 drives to finish. If I remove a drive and reformat it, will the raid 5 adapt to having 1 less disk? There will be plenty of space so it would be 3 disk raid 5.
This is not the best way to backup and fully relay on the RAID 5 to work degraded but yes doing what you want should work but you have to do it without the RAID trying to rebuild its self.
Before you remove one of the drives best to run Verify in RST so that all data is correct (and is good maintenance to do) before removing and drive to use as a backup.
Backing up with a RAID 5 degraded will be slow.
You can not go from a 4 disk RAID 5 to a 3 disk RAID 5 as a normal working RAID 5 with less space it can only run degraded unless you redo the array for a 3 disk RAID 5 setup.