upgrading hdd in ReadyNAS x4

Which firmware are you running?

Note that the archive disks are not designed for RAID use.

RAIDiator-x86 4.2.x has some expansion limitations that you will run into:

1. You cannot expand by more than 8TB over the life of the volume. So if the volume was 5.4TB when you last did a factory default you can't expand past 13.4TB

2. You cannot expand past 16TB. To get a higher volume capacity you would need to do a factory default (wipes all data, settings, everything) with the disks in place.

ReadyNAS OS 6 does not have these expansion limitations.

Well you should backup regularly anyway unless you don't care about your data. No important data should be stored on just the one device.

When you replace a disk in a single redundant volume the volume is unprotected till the resulting resync completes. If another disk is failing (disks can fail without any warning) and dies during the heavy stress of the resync then you could end up needing to restore from backup or consider purchasing data recovery which may ultimately prove to be partially/fully unsuccessful.

Some good things to do would include, downloading your logs (see the Sending Logs link in my sig - no need to send these in atm, but that articles shows how to download them) before adding the disk and keeping those logs in a safe place (they could be useful in the unlikely event something goes wrong)

You should also check the SMART stats of the disks. If a disk appears to be failing then obviously replace that disk first.

I would also suggest downloading the logs after a disk has been added to the array as that could also be useful in the unlikely event something goes wrong.