Why USB?  Would a NAS fill the requirement, providing access to disk shares over an ethernet network?  Plenty of brands exist, like Synology which can take drives of different sizes and do "the best it can" with raiding to get some protection while not being limited to the smallest disk capacity.

Additionally, multiple client computers can access the same share at the same time.

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If commercial products are a step too-far then home-brew solutions exist.  I ran FreeNAS for years, and samba in some form for decades and both need nothing more than reliable hardware.  A 10 year old box will run samba on linux perfectly well.