Quantum Spark appliances and regular Quantum appliances are different.
They will use USB, albeit in different ways:
1. Bootable USB drives with a valid Gaia OS should boot from said image (assuming ISOmorphic burned it to the USB correctly)
2. Quantum Spark appliances will accept a firmware image stored on the FAT32/XFAT volume on a USB drive. If present at boot, the appliance will be reimaged with the firmware image (replacing the backup firmware image) and will require a power cycle afterwords.
If you're having trouble with either of these options, please consult with TAC: https://help.checkpoint.com