could you try to simply 'dd' the iso to the usbdrive?:
`dd if=proxmox-ve_6.0-1.iso of=/dev/sdX bs=4M` (where /dev/sdX is your USB-drive)
That should work
sync afterwards or add the "conv=sync" to the dd command, else not all data could be synced over at the moment you unplug the USB pen drive.