Problems Fixed by Upgrading

Your SAMD21 or SAMx5x board UF2 bootloader may need to be updated to fix several possible problems:

• A problem with older bootloader BOOT drives not being seen by macOS versions 10.14.4 and later. This bug is fixed in v3.3.0-adafruit.10 and later.
• An intermittent bug that can erase parts of internal flash due to a power glitch (possibly just due to plugging in the board). This bug is fixed in bootloader v3.9.0 and later.
• A problem with BOOT drives being used or seen by ChromeOS, such as Chromebooks, and with Linux systems running firmware update fwupd versions 2.x.x or later. The affected Linux systems include Ubuntu 26.04 and Debian 13. This bug is fixed in v4.0.0 and later.

Updating Your Bootloader

To see if you need to update your bootloader, get the UF2 BOOT drive to appear as a mounted drive on your computer, in a file browser window. If you're running MakeCode, click the reset button once. If you're running CircuitPython or an Arduino program, double-click the reset button.

When you see the ...BOOT drive (CPLAYBOOT, FEATHERBOOT, METROM4BOOT, ITSYM4BOOT, PORTALBOOT, etc.), click the drive in the file browser window and then double-click the INFO_UF2.TXT file to see what's inside.

The example screenshots below are for a Circuit Playground Express. What you see for your board will be largely the same except for the board name and the BOOT drive name.

In the windows below, the CPLAYBOOT drive is displayed. The bootloader version is listed in INFO_UF2.TXT. In this example, the bootloader version is v2.0.0-adafruit.5.

If the bootloader version you see is older than v4.0.0, it is desirable or necessary to update. Depending on which operating system you are using, an update may not be absolutely necessary. See the Problems Fixed by Updating section above to decide if you should upgrade. For instance, the bootloader above should definitely be upgraded, since the version listed is old and has all the problems listed in that section.

Download the Update UF2

Download the latest version of the bootloader updater from the circuitpython.org Downloads page for your board. Below is the page for Circuit Playground Express. Find the Update the UF2 Bootloader section on that page and click the DOWNLOAD UPDATER UF2 button.

Copy the Bootloader Updater UF2 to the BOOT Drive

The bootloader updater file will be named update-bootloader-name_of_your_board-v4.0.0.uf2 or some later version. Drag that file from your Downloads folder (or wherever you saved it to your computer) onto the BOOT drive. The picture below shows dragging the file.

You could also use the command line to copy the file to the BOOT drive, if you are familiar with terminal commands.

After you drag the updater onto the boot drive, the status LED on the board will flicker and then blink slowly about five times. A few seconds later, the BOOT drive will appear in the Finder, Explorer, or file browser window. After that, you can click on the BOOT drive and double-click INFO_UF2.TXT again to confirm that the version has changed and you've updated the bootloader.