Personally, I have a SD/TF card in my GL-MV10002 Brume for sharing, just to keep things separate and be able to get much larger storage (32GB EXT4 format in my case).
If you want to share /data, it is quite easy:
chmod 755 means:
1st digit = 7 → Owner has Read, Write, Execute permission
2nd digit = 5 → Group has Read, Execute, but not Write permission
3rd digit = 5 → Everyone has Read, Execute, but not Write permission
This is a common default permission in Linux-based systems, if you do not want clients (guests) to delete files, so you (root) have to delete them.