After setting up my Pi4 as an NFS/PiHole server to replace a 3B+ which was doing the job, and serving the OSs for the other PIs via NFS, and realising my 500GB external drive was getting close to capacity, I decided to migrate everything to a 1TB (this was last week). My lesson from all this was don;t do stuff like this at 7am on a Sunday morning after you've been out Saturday evening.
I managed to trash not only the entire data on the 500GB, but also my backup, so music, videos and photos all gone, as well as the OS folders for the other PIs. Now, I've managed to recover from other sources (mobile phones have their uses, it seems), a couple of videos, but since were most ripped from my own DVD collection, apart from leg work, no problem, and I've even recovered my most precious photos.
However, the PIs have had to be re-built from scratch. One in particular I was turning over to a Nextcloud server exclusively. However, imaging to the NFS drive and updating repeatedly failed (no idea why, one of the other 3B+s is just fine), and I've eventually settled on the NextcloudPi image - now working but still on MicroSD. Waiting to migrate to NFS, but even getting this far has been a chore.
Suppose it's not helped by the media etc. being on the NFS drive connected to the Pi4, but even so, I seem to remember it being much easier 1st time around.