USB is unfortunately notoriously unreliable for long term storage connectivity.
I would not recommend leaving a USB drive with a pool on it connected and imported for long periods of time. USB connectivity typically works well enough for import, do what you need to do, then immediately export and detach. But anything beyond that is asking for trouble.
For a cheap, relatively reliable local backup, I’d recommend setting up another system on your LAN. Use whatever bottom dollar old gear you’ve got lying around or can acquire cheaply and easily–a J1900 Celeron with 4GiB RAM is plenty–or, if you prefer, a new cheap dev board with proper SATA connectivity such as an Odroid HC4.