Hi! Thank for the comment and a positive feedback! Due to the different pixel pitch, SL1 files are unfortunately not compatible with SL1S SPEED.
For the behavior you are describing with the USB drive and the files: the USB drive itself might be the culprit. Check that you are running the latest firmware on the printer. Check if there are special characters in the filename of the SL1S file.
The ".." /previous - root folder icon on top of the files list when you browse the USB drive on the printer screen may include the files from the root folder into the count; including the files stored locally on the printer. If the problem persists, please contact support via shop.prusa3D.com and provide them with the logs from the printer so the developers have something to look at :)