ReadyNAS Pro 6 no volume and all disks are red

Unfortunately, you made multiple mistakes here.  But if you didn't write anything to the NAS while it was syncing with the 10TB drive, then StephenB may be able to find you a way to re-mount your volume with that old 18TB included.  But the step after that is going to be to back up the data, so you also need to start thinking about how you are going to do that.  You didn't say how big your volume is, and the moderators haven't released your screen dumps that might show it, but if it's a lot, you may want to look into other brands of NAS and end up making that your primary and relegating the Pro6 to backup duty.  Even without operator error, RAID is not sufficient to keep you from losing those valuable memories.

There is an old IT saying that still holds:  If you only have one copy of something, you must not think it's important.

My personal solution is this:  All data is backed up to another NAS locally (for speed).  Really important stuff (photos, documents, most recent PC backups) are also remotely backed up to a NAS at my brother's house 3 states away (in case of fire, flood, hurricane, theft, etc.).  If you don't have a way to do remote NAS backup, a cloud-based service is an option.  For smaller volumes than I assume you have based on at least a couple 18TB drives in your NAS, just a cloud service may be enough, since they have their own backup scheme.  Or if a lot of it is music, videos, etc that can be recovered from the original source, just don't back that up and deal with the time it takes to re-create the NAS files if needed.