Maybe even make it modular. Sell something that looks like a thumb drive but has a slot in the back that you can plug a small security key into (think something with a form factor like a YubiKey Nano).
When a security key is plugged into the slot the thumb drive provides storage for the security key and appears to the computer as a security key.
When a security key is not plugged into the slot the thumb drive functions as an ordinary thumb drive.
You would ordinarily keep the security key plugged into the slot, but if you ever decided you needed more storage you could by a bigger storage module, remove the security key from your old storage module, plug the old module into the computer, copy the encrypted files, plug in the new module, copy the files to it, then plug in the security key.
When a security key is plugged into the slot the thumb drive provides storage for the security key and appears to the computer as a security key.
When a security key is not plugged into the slot the thumb drive functions as an ordinary thumb drive.
You would ordinarily keep the security key plugged into the slot, but if you ever decided you needed more storage you could by a bigger storage module, remove the security key from your old storage module, plug the old module into the computer, copy the encrypted files, plug in the new module, copy the files to it, then plug in the security key.