I'd like to use my hAP ac3 as a poor man's backup solution (in addition to other local & cloud copies).

I installed ROSE package and hooked up an external 5TB drive then formatted it to ext4 with GPT to be able to use the entire drive as a single partition. Initially, the drive would show up under "Files" menu and I could see any files in router's NAND storage (config files, etc) and also files on HDD that I copied there via SMB. Since everything looked good, I copied about 50,000+ files to the HDD, but now no files show up under "Files". Can't see files in NAND or files on HDD.

Since Files menu typically lists out all files, my guess is there are too many files for router to display and instead doesn't show any. I can still access files on HDD via SMB.
Any way to at least see files in NAND?

Disconnect that Rose storage drive.
Does NAND content appear again in Files ?

If so, might be worthwhile to contact support because it is probably a bug.
My educated guess (but could be totally wrong): some internal variable with wrong size definition. 5Tb is a huge drive for a Mikrotik