Step 1. Hook your switch SD up to your computer (Hekate's SD Tools UMS mounting, memloader with tegrarcmgui, or just plug your SD card into your computer all work.)
Step 2. Copy all the files from your SD card to your computer, place them in a folder for now. Somewhere you'll know they are, I'd put them on your desktop in a folder.
Step 3. Download guiformat and open it. Select your SD card with the drop down, leave allocation unit size as the default
32768, make sure under format options
Quick Format is enabled (if it isn't, the format will take awhile.)
Step 4. Copy your files back to your SD card once the format is finished.
Step 1. Right click your task bar, and select
Step 2. Click on
More details >
navigate to the
Step 3. Find
explorer.exe in the list, click on it, and then click on
End task in the bottom right. Hit
yes to the confirmation. Note, if you had ongoing copies or stuff using explorer, this will break them. Only do this if you don't have anything else going on.
Step 4. Attempt formatting your SD card again with guiformat (should have left this open.)