When I was moving from a Windows NAS (God, fuck windows and its permissions management) on an old laptop to a Linux NAS I had to copy about 10TB from some drives to some other drives so I could re-format the drives as a Linux friendly format, then copy the data back to the original drives.
I was also doing all of this via terminal, so I had to learn how to copy in the background, then write a script to check and display the progress every few seconds. I'm shocked I didn't loose any data to be completely honest. Doing shit like that makes me marvel at modern GUIs.
Took about 3 days in copying files alone. When combined with all the other NAS setup stuff, ended up taking me about a week just in waiting for stuff to happen.
I cannot reiterate enough how fucking difficult it was to set up the Windows NAS vs the Ubuntu Server NAS. I had constant issues with permissions on the Windows NAS. I've had about 1 issue in 4 months on the Linux NAS, and it was much more easily solved.
The reason the laptop wasn't a Linux NAS is due to my existing Plex server instance. It's always been on Windows and I haven't yet had a chance to try to migrate it to Linux. Some day I'll get around to it, but if it ain't broke... Now the laptop is just a dedicated Plex server and serves files from the NAS instead of local. It has much better hardware than my NAS, otherwise the NAS would be the Plex server.