It's not a problem at all, unless you're using rekordbox 4.
I use an external SSD as my master database AND music storage location. I can use that same SSD or another USB drive as an export device (which I often do, so I don't have to take the master drive with me to events).
The beauty of rekordbox 5 is it no longer exports a duplicate to the drive if that drive is also the source; it simply links a pointer to the original file.
My suggestion is first to clean up your collection to ensure everything is on the external drive so you could move between computers and have the same collection of music everywhere that drive goes. Move the physical audio files from the internal to the external, then use the mass-relocation tool to point the database to the new location of those songs.
It will be the source of your collection as well as visible as a device.