I assume, its a standard hard disk installation and not a live installation on your USB hard disk.
Plop Boot Manager: The problem is, that your computer seems to use an internal USB hub, witch is not supported by the 5.x release.
Plopkexec: I assume, you use grub2 as boot manager on the USB drive. grub2 config files are not well supported by plopkexec. You should create a custom plopkexec.cfg file with kernel and initrd configuration in syslinux format.
Best regards
Elmar