Removing disks from a volume, and using a "global cold spare"
I am using a Netgear RN316, with 5 4TB disks, and a 2TB disk, in one RAID-X volume. (I could not identify the exact 316-model).
I want to perform the following:
1) Remove the 2TB-disk from the volume (and physically put it in a drawer...).
2) Remove the 4TB-disk from slot 5 in the volume, and keep it aside as a backup disk. I thought all this would be easy to perform, switching to "flex-disk". But I could not do anything at all to get to where I want with my disks:-( What wouldbe be the proper way to perform the above?
I 1) Remove the 2TB-disk from the volume (and physically put it in a drawer...).
2) Remove the 4TB-disk from slot 5 in the volume, and keep it aside as a backup disk.
There's no way to remove both without losing all the data on the volume. You have single redundancy, if you remove two disks the volume is immediately lost. XRAID does not support downsizing the array. You can expand it, but not shrink it.
Also, you can't use one of the disks as a backup. Data and Parity blocks are spread across all the disks. If you remove one, the volume is degraded, but your data is still ok. If the array changes after that (because you update, add, delete files), then if you attempt to add the removed disk back the system will detect that the array is out of sync. And since the data is spread across the disks, you can't recover anything useful from just one disk.
Re: Removing disks from a volume, and using a "global cold spare"
You cannot shrink ReadyNAS volumes. So, as @StephenB says, you can't do what you want to. Removing one drive makes your volume non-redundant. Removing two makes it dead.
I'm not sure how you intended to use the removed 4TB for backup. "Universal spare" is certainly not what you are talking about. If you planned to re-purpose it using a USB dock or similar, then you can use it -- but you have to backup your data, destroy the volume, and then start over with the rest of the NAS. You could remove the 4TB (but not also the 2TB) and use the removed drive for your backup via a USB dock (assunming it's big enough for the amount of data you have) at some risk, because your volume will be non-redundant through the process.
You can (though it's not recommended) configure the 4TB as a separate volume, back up to it, and remove it. It is best to use the "Export" function before removal. Of course, that single 4TB will not have enough space if the rest of your NAS is full. The constant removal/insertion that a backup would likely get to stay up to date will be hard on the NAS. You could also just leave the separate volume in place, though there are failure modes that could take out both the primary and backup volumes in that case. Again, you'll have to destroy the main volume and re-create with fewer drives after you've backed up.
