I wonder how I should encapsulate a bunch of old, internal SATA drives in a single black-box (literally) whose output would be a single USB plug. It would present itself as a single drive with the total size combined. Is that possible? Would it require an external power supply?
I use this setup for Chia mining.
Get a PC case, a PC power supply, a 1X to 16X PCIe Adapter for GPU Mining to power a HP SAS Expander Card 24-Port SAS PCI-E Expander Board 468405-001. The all green ones, avoid the earlier version with yellow writing.
Then get any SAS controller like a LSI 9207-8e in IR (integrated raid) if you want one drive or IT (integrated target) mode if you want to see individual drives.
Note the HP board only uses the PCIe for power, so no need to connect it to a motherboard in the case. The mining 1x to 16x will take power from a 4 pin power from the power supply.
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.
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.