You are better off booting from an SD card and using the USB drive for the system (USB install in the installer). See here: Boot OSMC from USB mass storage RPi3B+ - #13 by grahamh
Just to be clear: OSMC supports having the boot partition on SD card (250MB) and the rest of the system on USB. The installer does not support having both the boot partition and the system partition on USB. It can be done but you need to know what you are doing.