Hmm, the /dev/... folder contains the direct hardware connectors, you shouldn’t be able to store something there and cannot copy data FROM there. From there you can mount drives the the actual mount points, which are/should be located inside /mnt/....
You could check your fstab where the drive is really mounted to or add the related entry to mount it to a desired location: cat /etc/fstab
Or use df to get an overview:
root@micha:~# df
Filesystem 1K-blocks Used Available Use% Mounted on
...
/dev/sda 2884154032 219167796 2518456524 9% /mnt/sda
...
- Hardware connector
/dev/sda, mount point where I can store to and open files from:/mnt/sda
So I guess, /dev/sdb is mounted to /mnt/sdb, which would be at least the usual way/location. Now the question is, if you want the whole drive as Nextcloud datadir or just a subdirectory /mnt/sdb/ncdata.
cp -a /var/ncdata/. /mnt/sdb # to use the whole drive, or
cp -a /var/ncdata /mnt/sdb/ # to use `/mnt/sdb/ncdata`
€: Ah okay, if it’s working now and the data is on the correct drive, do not mess around, the above is just cosmetic then 
.