First of all thanks for the help.

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So I made a 5GB disk image and mounted it successfully

crosh> vmc create-extra-disk --size=5G --removable-media "SD Card/extra-disk.img"

crosh> vmc start termina --untrusted --extra-disk "/media/removable/SD Card/extra-disk.img"

lxc config device add penguin extra_disk_0 disk source=/mnt/external/0 path=/mnt/external/0

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And I used nano to write a text file to the disk

user@penguin $ sudo nano /mnt/external/0/test.txt

And it writes and saves! So that a huge win.

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But I tried restarting my chromebook to see if I need to set it all up everytime and I need to use

crosh> vmc start termina --untrusted --extra-disk "/media/removable/SD Card/extra-disk.img"

to turn on crostini. The disk is still mounted, but my text file isn't there anymore. Any ideas how I can remedy this?