I have two RPi’s both running DietPi. Both of them have two USB drives plugged in. One is a SanDisk and runs DietPi, the other is a Samsung and is mainly for backup purposes.
One of them tries to boot from the ‘backup’-USB. The other RPi doesn’t have that problem. It’s a recent problem, it worked fine in the past. I’ve checked the cmdline.txt if the correct partuuid is used, both have the correct USB-drive in there.
I do not understand why one fails to booth with that 2nd USB-drive inserted.
I guess you mean the RootFS is located on your external USB device? Correct? Did you checked /etc/fstab if settings for RootFS are correct as well? Next to that, you could have a look to dmesg. Right at the beginning you should find something like this: