[Milhouse]

EDIT: Checking the ext4 partition, it just has an empty "lost+found" folder on there but maybe that was created when I formatted the partition and doesn't indicate that the RPi was able to access it.

Yes, that's always created when the filesystem is added. Did you label the partition/filesystem with the name "Storage", just in case that's an issue.

Assuming the filesystem is valid I'm not sure why it wouldn't boot, is your cmdline.txt referencing the partition as "disk=/dev/mmcblk0p2" ?

When you drop into the debug shell after a failed boot you could try mounting /dev/mmcblk0p2 manually and you might see what the problem is.