I have V88 pro and I tried all usb slots and none of those booted libreelec.
Then I created a dummyfile named update.zip with windows notepad which contained only one space and copied it to BOOT partition.
Then booted my v88 pro to android and went to update&backup clicked select and chose sdcard(Boot-partition)/update.zip and then update.
Now my tv box boots libreelec when the sdcard is inserted. If not it boots android.