Prusa error 13703 (Unknown File System) appears on the MK4 and its equivalents are #26703 on the MK4S, #27703 on the MK3.9S, #21703 on the MK3.9, and #31703 on the CORE One. The printer detected a USB drive in its port but cannot read it because the drive uses an unrecognized file system format.

What the Error Means

Prusa printers require USB drives to be formatted with the FAT32 file system. Modern computers often format drives larger than 32 GB as exFAT or NTFS by default — formats that are common on computers but not supported by the printer’s USB host controller. Drives that came pre-formatted from the factory, newly purchased drives, or drives that were reformatted on a Mac (which defaults to exFAT or APFS) are frequent offenders. The fix is simple: reformat the drive as FAT32 on your computer before using it with the printer.

Step-by-Step Fix

1. Identify the current file system of the drive. On Windows, right-click the drive in File Explorer and select Properties — the file system is listed in the General tab. On macOS, open Disk Utility and select the drive to see its format.
2. Format the drive as FAT32 on Windows. Right-click the drive in File Explorer, select Format, choose FAT32 from the File system dropdown, and click Start. This option only appears natively for drives up to 32 GB.
3. Use Rufus for drives larger than 32 GB on Windows. Download Rufus (a free, open-source tool), select your drive, choose FAT32 as the file system, and run the format. Rufus bypasses Windows’ 32 GB FAT32 limitation.
4. Format the drive as FAT32 on macOS. Open Disk Utility, select the USB drive (not a partition), click Erase, choose MS-DOS (FAT) as the format, and confirm.
5. Copy your files back to the drive and re-insert it into the printer. After formatting, transfer your G-code or firmware files to the drive root and retry.

Parts Required

• No replacement parts are required for this fix — this is a drive format issue
• Replacement USB flash drive (32 GB or smaller preferred, for native FAT32 compatibility without third-party tools)

If the drive is formatted as FAT32 and the error still appears, try a different USB drive — some drives have compatibility issues with certain USB host controllers.

Frequently asked questions

What does Prusa 13703 mean?

Prusa error 13703 (Unknown File System) appears on the MK4 and its equivalents are #26703 on the MK4S, #27703 on the MK3.9S, #21703 on the MK3.9, and #31703 on the CORE One. The printer detected a USB drive in its port but cannot read it because the drive uses an unrecognized file system format.

How do I fix Prusa 13703?

Prusa error 13703 means the USB drive uses an unrecognized file system. Learn how to format the drive as FAT32 to make it readable by your Prusa MK4-series printer.