I have a 4-bay Synology with a 4x1TB RAID-5 volume that has been up and running without issues for 3-4 years. Of the slightly less than 3TB capacity about half is used.
Now, because RAID-5 doesn't really cut it, and 4 disks take 4 times as long to spin up and be checked when the disk station powers up in the morning (it turns on automatically on schedule at 7), plus noise, plus power consumption, and whatnot...
... after finding a still-sealed 3TB disk that I had bought half a year ago and forgotten about in the desk drawer, I've impulsively bought two 4TB drives with the idea of replacing the 4-disk RAID-5 configuration with a 2-disk setup.
Two 4TB disks, that's double the capacity (not like I really need it, but still) and only half the number of drives. Great.
Now, the obvious problem is that preferrably, the transition should happen without
- losing all (or, well any) data
- having to copy 1.5TB worth of medium and small files over GbE network and back
...either one being a major nuisance. Sadly, I know only too well what ordeal the network copy would be because I have a second disk station that pulls backups of the most vital data (not all) once per week -- it takes forever.
So, the idea was to pull out one disk from the RAID, freeing one bay. That'll work ("disk failed") thanks to redundancy.
Then plug the spare 3TB disk in, copy all data over on the device, destroy the old volume, replace disks, create new volume, copy data back. Take out spare disk (or leave it, whatever).
That won't work, as far as I can tell. Plugging in the spare disk will make it part of the RAID (but using only 1/3 of its capacity).
Somehow, I'd need a way to either convert a 4-disk RAID into a 3-disk RAID (hardly possible?), or a way of telling the disk station that the new disk that I'm plugging in is not supposed to replace a failed disk, and is not supposed to be filled with parity.
Is there a way of achieving this? Basically I need something like an emergency copy with a one-disk-failed array (onto a disk, pretending I don't have one). Something the like?