The Original Prusa MINI uses USB flash storage to read G-codes during printing. It can happen that the USB drive is not recognized when inserted and the display will say "No USB", even when a USB is inserted. Since this issue can have several different causes and culprits, there is unfortunately not one universal way to fix it. Here we will go through some of the steps to do and things to check to resolve this.
If the USB drive is not recognized when inserted but becomes visible after restarting the printer, it is most likely a firmware issue. Handling of USB storage media is improved with each update.
Some languages have special characters or diacritics that cause issues when attempting to read the drive. This has also caused the Watchdog error when attempting to print. Therefore, make sure you only use standard Latin letters in upper and lower-case. This is also why you will find that PrusaSlicer will autogenerate filenames with an underscore (_) instead of spaces. It is not always necessary, but to ensure backward and future compatibility, we recommend following these guidelines.
Filenames do not have a set maximum length of characters. However, they may not display correctly if they are very long. Therefore, make sure you do not make filenames unnecessarily long.
The same restrictions apply to the type and length of characters in folder names. Moreover, it is also good to know that a folder tree can only be 10 folders deep, i.e USB/Folder1/Folder2/Folder3/Folder4/Folder4/Folder6/Folder7/Folder8/Folder9/Folder10/
The printer will ignore files that it can not read. However, since the printer's firmware is updated using the USB drive, it can happen that you will have multiple firmware files on the drive at any given time. This can cause conflicts, so make sure you delete these and try to only have them present for the flashing process, then delete them.
The easiest way to test this theory is to try with another (preferably new) USB drive, formatted to FAT32. ExFAT is not compatible. However, there are some procedures you can try with your current USB drive.
The filesystem can become corrupted when you pull out the stick without unmounting/ejecting it. Most OSs have built-in features for "Recovery" or "Repair". There is also a range of third-party software with this functionality. However, the procedure brings few benefits, besides keeping your files and formatting, compared to if you reformat the drive. It is also a slower process.
This will clear all the files on the disk and build the filesystem anew. Only FAT32 is supported! For drives over 32 GB, you will need a 3rd party application. For more information on formatting USB drives and SD cards, please see SD cards and USB drives.