The printer won't read the SD card or USB drive

Symptom

The printer doesn't detect the card or USB drive, doesn't show the files, or warns that the file is incomplete or corrupted. Sometimes it only fails with one particular drive or certain files.

Likely causes, from most to least likely

1. Incompatible format: many printers (Prusa's, for example) only read FAT32, not exFAT, with an MBR partition scheme, not GPT.
2. The card is locked with its side switch, not fully inserted or inserted the wrong way.
3. Incomplete or corrupted file: the drive was pulled from the computer without ejecting it, the export was cut short, or it was removed while the print was starting.
4. A file name with special characters, such as accented letters, apostrophes or other alphabets.
5. A damaged drive (flash memory can become corrupted without warning) or a dirty or damaged slot or port.

Checks to tell which one it is

• Put the drive in your computer: if even the computer doesn't see it, it's probably damaged.
• Check the file system on the computer: it must be FAT32. On Windows, the built-in tool won't format drives of 32 GB or more as FAT32.
• Check the SD card's lock switch and that it goes all the way in, the right way round.
• Try another freshly formatted drive and a file with a simple name.
• If the error happens with every file and drive, check the slot or port: no dust, no debris and no damage.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Check the lock, orientation and cleanliness

   Unlock the card, dust off the card and the slot, and insert it fully the right way round.

   Check: The printer detects the card.

   If that does not fix it: Format the drive.

2. Format as FAT32 with MBR

   Back up your files first: formatting erases everything. Format the drive as FAT32 with an MBR scheme. On Windows, drives of 32 GB or more need a separate program; be careful not to format another disk. An 8 to 16 GB drive is usually plenty.

   Check: The printer shows the files.

   If that does not fix it: Copy the file again.

3. Copy the file again properly

   Export the full G-code again from the slicer, copy it, safely eject the drive before removing it, and use a name without accents or symbols.

   Check: The file opens without incomplete-file warnings.

   If that does not fix it: Try another drive.

4. Try another drive

   If a new drive formatted as FAT32 works, the old one is damaged: save your files and replace it.

   Check: The printer reads the new drive without problems.

   If that does not fix it: If it won't read any drive, the slot or the board's USB port may be faulty: contact your manufacturer's support.

Prevention

• Always safely eject the drive before removing it from the computer.
• Use simple file names without accents or symbols.
• Keep a spare drive: flash memory can fail without warning.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• SD card not working · Prusa Research
• SD cards and USB drives · Prusa Research
• USB not connected · Prusa Research
• File incomplete. Continue anyway? · Prusa Research