resinch a dieing disk or add new disk & resinch?

If you reboot it as it is now, it shouldn't rebuild anything. It will do a volume scan, and attempt to repair any damage it sees with the file system (such attempts can sometimes result in data loss). But it won't rebuild the RAID array (in fact it can't).

Then I am TOTALLY confused.
When I rebooted the NAS (with HD1 "paused", if I remember correctly), after the reboot, the dashboard did NOT come up as usual. Basically I could not do anything, see the logs etc.
So I checked the NAS, found it re-synching, and powered it off immediately.

Just to clarify, what is the difference between:
Volume Scan
Re-synch
RAID Rebuild

and what would I need, to end up without (if possible) any data loss, like it was 15h ago?

Also, how do I invoke a RAID rebuild (if this is what I need)?

If there's data to copy off, then do it when it boots. Then insert your spare disk into slot1

But when I will reboot NAS, will it not start re-synching (like it was doing already), and correcting errors (and maybe incurring in data loss) ?
Besides, even If it did only a "quick" volume scan" (but how could it possibly do that, with HD1 missing?), I would not have the space to copy the ~5 - 5.5ish TB of data.

I feel like i am going around in circles.... :-/