A MicroSD card is required to be inserted into your board for offline mode. This applies even for cases where data is only intended to log to USB serial. 

Which MicroSD Card Should I Get?

Any MicroSD Card with a capacity of less than 32GB should be good. When purchasing the card, be aware that some cheap cards are "fakes" and may cause headaches. The MicroSD cards in the Adafruit shop are guaranteed to work.

You will also need a way to read the logged data from the MicroSD Card. To do this, you will need an external SD Card reader to plug into your computer.

Formatting under Windows/Mac

If you bought a MicroSD card, chances are it's already pre-formatted with a FAT filesystem. However, you may have problems with how the factory formats the card, or if it's an old card, it needs to be reformatted. The Arduino SD library used in the Wippersnapper software supports both FAT16 and FAT32 filesystems.

If you have a MicroSD card with a very small capacity, say 8-32 Megabytes you might find it is formatted FAT12 which isn't supported by Offline Mode. You'll have to reformat these cards.

Either way, it's always a good idea to format the card before using it, even if it's new! Note that formatting will erase the card so save anything you want first.

Before proceeding with the guide, we strongly recommend you use the official SD card formatted utility to format your MicroSD Card. This tool is written by the SD Association and it solves many problems that come with bad formatting. The link is below:

Insert the MicroSD Card

The Feather RP2040 Adalogger's MicroSD card slot is located at the end of the board, directly behind the STEMMA QT port. After formatting the MicroSD card, insert the card into the MicroSD card slot.