Q: Why should you test your SD card?

A: There's a few reasons, such as when you buy a new SD card to make sure it isn't fake. Or if you were having issues with installing games, and they kept corrupting themselves, it's a way to make sure your card isn't going bad and helps to narrow down issues.

Note that "full formats" for FAT32 can help, however they're not an automatic "fix it all" solution. A quick format doesn't check for bad sectors, where a full format (slow format often called) does check sectors, however you don't know how many bad sectors there might be, or what condition your drive is in making it pretty pointless. Using a tool like Flash Drive/Card Tester will show you how many bad sectors you have, giving a better idea if the card might still be salvageable, or if you should just replace it outright.

Note: If you were using your SD card before, this will erase everything on your card. Back up contents you want from it before doing this, I am not liable if you lose your saves because you didn't back them up. See this part of my guides for how to back up different stuff on your switch.

Requirements.

A Mac with homebrew installed.

An SD card with (ideally) nothing on it.

Installing Homebrew on your Mac.

Step 1. Launch Terminal.

Step 2. Paste this command into Terminal (it's from the homebrew site here.)

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

Enter your password if it asks you to, it will take a moment or two to install. You can verify you installed brew properly after by typing brew, and it'll give you a list of commands/arguments you can use.

Installing F3.

Step 1. Launch Terminal and install F3 with brew using the command below.

brew install f3

Testing your SD card with F3.

Step 1. Launch Disk Utility so we can find out what Volume your SD card is.

If you haven't already, select the drop down tab in the top left of Disk Utility, and select Show All Devices.

Step 2. Select your Volume on your SD card. Note down the reported mount point, in my case it shows up as /Volumes/TEST NX.

Step 3. Tab back into Terminal (or launch it again if you closed it.) Type out f3write "/Volumes/[YOUR VOLUME HERE]", hit enter for it to start. This will take a bit depending on the size and speed of your SD card. Make sure to include the brackets.

Once it's done F3 will tell you the average write speed and your Terminal should look something similar like this (or possibly more parts, if your card was larger than mine.)

Step 4. Tab back into Terminal (or launch it again if you closed it.) Type out f3read "/Volumes/[YOUR VOLUME HERE]", hit enter for it to start. This will take a bit depending on the size and speed of your SD card. Make sure to include the quotation marks.

Once finished, your results should look similar like the above image if your card isn't corrupt and is fine. If the utility detects corruption, you can try formatting your card as FAT32 with Disk Utility, and then test it once more. But your card most likely is failing, or faulty.

You may also now delete the hardware test files the command line utility made on your SD card, make sure your empty your Mac's Trash after too.

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