Ready NAS not responding after resync

mdgm wrote:
Sounds like you have a bad disk. Can you power down, remove your disks (label order), and check e.g. SeaGate disks with SeaTools and WD disks with WD LifeGuard Diagnostics?

I've noticed that it managed to shutdown. Not sure how. I've pressed the button for a long time, and after I came back a few minutes later it was powered off.
I rebooted it and it did a Quota Check, then managed to send me a few emails (more errors from the same disk) and I eventually got an other email that a resync has just started again. But it does not respond to anything now.

Now: should I try to (re)boot it to perform a backup or just remove the candidate bad disk (it is always the same) and get a new one? Or run a backup without the broken disk?
Unfortunately I do not have any PC to perform the checks, I will trust the emails.

And should I remove the disk while it is powered off or as soon as it boots?