As far as i could tell, it was derived from me extracting rom zip files inside the 64gb pen drive when it was connected to my pc.
Since I completely reformatted both micro sd and pen drive, and later recreating the filesystems (and placing the the already uncompressed roms in their folders), everything started to work perfectly.
Maybe some "trash" data conflict? Anyway, learned to be more careful about this in the future!, but, wanted to share this little event for reference sake :)