I finally fixed it using the instructions in this answer.
Unmounting the partition and then using this command seems to have fixed it, though I'm not sure what exactly made it work this time.
sudo mount -o rw,users,umask=000,exec /dev/mmcblk1p1 /media/deso/Library
I can now create folders and files in the SD card, and Steam accepted the drive as a library without any issues.
I didn't realize you were dual booting. Windows fast boot and hibernation both tend to lock drives that have been introduced in windows in order to access them more quickly. You may also want to ensure secure boot is off in the bios. It may cause havoc with Linux drivers.
I'm glad you found a solution.
How would I boot into Linux with secure boot enabled?
Anyway, while the card works fine now, removing it and inserting it again created a second mounting point called "Library1" (original was Library), and both of them show up in the files explorer.
Do I create a new issue?
You can test using Zorin OS with Secure Boot enabled - you may need to set up some mok-utils... But most users don't have too much trouble.
The issues caused by Secure Boot stem from the fact that Microsoft signed off on most Linux applications to allow Secure Boot to pass them through - but not all applications.
Users not using those unsigned apps have no trouble.
Those that do happen to use those unsigned apps run into trouble if they are also using secure boot.
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