NAS540, FW V5.21(AATB.13), "manage" option greyed out, can't edit RAID settings

peejay

hello!

I had two disks, 6TB each, configured as RAID1. after years of service, one changed it's S.M.A.R.T. status to "bad", so naturally - I've bought a spare one, exact same model to replace it.

because this NAS contains an years-long archive of work, and also is used on a daily basis, I was "too afraid" to just take out one disk so I've put the third one into NAS, and added it to RAID. I guess that was probably a mistake ;)

now, I'd like to remove the "bad" disk [it still works, but I just want to remove faulty one, and free it's bay to have two empty again [I'm planning second raid, so I need those two spare bays].

I thought I just go to the Storage Manager/Internal Storage/Volume click on the volume, click on the "Manage" option and degrade RAID to use just two disks but… the "Manage" option isn't available…

is it by design, or I did something wrong?
can I and if so - how do I get rid of the faulty disk and in the same time - "shrink" the RAID to just two disks, without risking any data loss?