I do this when I go traveling.
NAS music is on \\NAS\music\
Portable drive is M:\music\
Use a file sync tool like FreeFileSync to do a mirror sync to copy your music to the portable drive with all folders and folder structure.
In JRiver Media Center select all of your files.
Then open the "Rename, Move, & Copy Files" dialog (Tools >> Library Tools >> Rename, Move, & Copy Files...)
We want to do a find and replace to get the library pointing to the files on the portable drive instead of the NAS
Wiki page for the Rename, Move, & Copy Files dialog:
https://wiki.jriver.com/index.php/Rename,_Move,_and_Copy_FilesIn the top dropdown select "Update database to point to new location (no file rename, move, or copy)"
Clear all of the other checkboxes except for "Find & Replace"
Find What: \\NAS\
Replace With: M:\
Then when you get home you do things the other direction. Use FreeFileSync mirror to copy the music folder from your portable drive to the NAS.
Use the Find & Replace with
Find What: M:\
Replace With: \\NAS\
Be very careful when using sync tools like FreeFileSync. It is too easy to accidentally do things in the wrong direction and overwrite and destroy your source. So doublecheck everything before you hit the "go" button. And have up to date backups of the NAS just in case. And while traveling have two portable drives with one acting as a backup.
I do things this way when I travel. It works for me. All of the tag updates and file changes you make and file additions and file deletions will get transferred to the NAS when you get back. You can even buy new music while on sabbatical and those new files will get transferred to the NAS when you return.
I would suggest formatting your portable drive as NTFS. Because NTFS handles long filenames and long paths better than FAT32. That way you won't end up with truncated filenames or lost files because of names or paths that are too long when you sync to the portable drive.