This is why people say not to use USB for permanent storage. But, to answer the question:
From memory, “nofail” means the machine continues to boot if the drive does not show up which explains why it’s showing up as 100GB: you’re seeing the size of the disk mounted to / .
If the only purpose of these drives is to be passed through to Open Media Vault, why not pass through the drives as USB devices? At least that way only OMV will fail and not the whole host.
Why USB? Can the drives ve shucked and connected directly to the host or do they use a propriety connector to the drive itself that prevents that?