Older versions of Adafruit's nRF52840 boards were shipped with a bootloader that does not handle large UF2's, including CircuitPython 8.2.0 and later, and has other issues. To check whether you need to update the bootloader, double-click the reset button, and look in the ...BOOT drive for INFO_UF2.TXT.

Inside that file, check the version number of the bootloader. There are multiple version numbers. Look for the number just after "UF2 Bootloader" on the first line. See the picture below It should be 0.6.1 or newer.

The dafruit nRF52 Bootloader can be upgraded/downgraded without any additional hardware. There are 3 different ways to update bootloader, each has its pros and cons:

• Use UF2: This is the fastest and safest way to update bootloader. However, it requires your existing bootloader version is at least 0.4.0 and only work with nRF52840 (not nRF52832).
• Use Arduino IDE: work with all Adafruit nRF52 boards and bootloader version, typo free but may not be the latest bootloader version due to the BSP release cycle.
• Use Command Line: work with all boards and bootloader version (including 3rd party one). This command line uses the back-end of the Arduino IDE method above.