I wouldn't try to remove two drives from that set anyway. It's too risky to play with adding / removing / upgrading drives in a RAID. My advice is
1) Get an external drive
2) Do a backup to it
3) Verify the backup.
4) Break the RAID and scrub the drives
5) Build a new, empty RAID and restore to it.
Thanks, it's a 6TB volume and I only have 2TB of external storage. I can't afford any additional HDD's and all my critical data is already backed up. There's about 3-4TB of non-critical tv recordings that I'd prefer to keep but maybe I'll take a risk and guess at which two are the mirrored drives.