I suspect the bootloader is enumerating each drive and trying to boot OSMC installer from it. This could cause some delays with so many drives attached.
You could run this command via SSH:
sudo fw_setenv osmcfromusb "true"
Keep in mind that you will not be able to reinstall OSMC from a USB stick again (SD will still work); unless you use a non-conductive pin on the recovery switch inside the box.
Sam