ASK NC

Hey guys I hope you are all doing well. I need a little help with a change I want to make on my NAS

NAS: TS-653D (QTS 5.0.1.2173)

4 x 4TB disks in RAID5 (Slots 1 - 4)

Single storage pool 8.5TB unallocated

1 Thin Volume 7TB free

I would like to permanently remove the disk from slot 4 to use for another purpose. Is there a way to safely reduce the storage pool to achieve this?

You can technically remove the last drive. But this will mean no redundancy. There is no way to change the number of drives in existing RAID from 4 to 3. You will need to delete and recreate a RAID.
You can then have RAID5 on first 3 drives. The last drive would have basic mode - running on its own.

I hope this helps.