16TB should work, attach it with no data and see if Roon will accept it. Suspect you should format as well (not sure if it is possible to format external USB drive from web interface of the Nucleus?)
Then it should be just to connect it. I would avoid removing the NAS share (follow the disable concept described above) if history is important to you.
I recommend formatting as NTFS if you use such a large disk. ExFAT is very ineffecient using large block sizes compared to NTFS, meaning that you do not hae as much hard drive capacity available to you. Roon supports NTFS drives and it should be compatible for reading across most operating systems.