Okay, here's the full message
Kernel panic - not syncing: VFS: Unable to mount the root fs on unknown-block(0,0)
The line before that says:
Please append correct "root" option: Here are the available partitions
And then lists the partitions. It shows my flash drive and the two partitions of the one physical drive.
*EDIT* I tried the USB drive again on my desktop. It got the same error even though it worked fine the last time. I did a clean install on the USB, recreating all the steps I could remember, but still got the error!