I was able to fix the problem! I continued searching around and stumbled across this forum topic about coreboot: Librem coreboot Utility Script: usage, discussion, and help. I noticed that someone else in the thread was getting the exact same error message from sudo bash ./coreboot_util.sh that I was:
Unhandled programmer parameters: ich_spi_mode=hwseq
In the thread @MrChromebox mentions that the build utility should be updated to address that error; I followed his link to the build utility on gitlab (https://source.puri.sm/firmware/utility) and saw this message:
Project 'coreboot/utility' was moved to 'firmware/utility'. Please update any links and bookmarks that may still have the old path.
I noticed in the bash commands to download the script the old path is still written out, so instead I changed it to
wget https://source.puri.sm/firmware/utility/raw/master/coreboot_util.sh -O coreboot_util.sh
which downloaded fine. After running the new build script and updating my firmware, pureboot was able to read the USB without problems.
I’m not sure if any of the steps I went through above actually fixed the problem, or if it’s all a coincidence. but I figured I’d write out what I did in case anybody else is having a similar problem.