Most of your SD cards or USB thumb drives have a single partition, but maybe you've used one in an interesting way, like for a Raspberry Pi or for a Chrome OS recovery image and now it has partitions. If you format the device on Chrome OS it will only format the partitions - it won't re-partition back to full size.
For a good time, write a FreeDOS image to an SD card and then format that on your Chromebook. Now you have a 30MB filesystem on your 32 GB SD card. Yay!
Again, we're going to use Google's Chromebook Recovery Utility.
- Open the app and click on the ⚙️ icon
- Select "Erase recovery media" (it doesn't care if your media has been used for recovery)
- Select the media you'd like to erase and click "Continue"
- Click "Erase now" if you're sure
- Remove and re-insert the device
- Browse to the device in the Files app and click "Format device"
Obviously this won't let you configure any kind of advanced partitioning scheme. If you need that, you'll need another OS (for now).