If a pool is unmounted then the share, which is a subvolume of the volume (pool) is most likely also not available. We do mount them in addition to the main pool top level but a poorly pool will often affect some or all of it’s shares (subvolumes).
The pools are auto mounted at boot, we don’t yet have an unmount / mount option within the Web-UI.
The drive may have become disconnected. This can happen with USB as it is a rather flaky interface, both in hardware and software. Often resorting to power cycling, or hard resetting it’s devices at the drop of a hat.
Best to look in the main system journal at the time it became disconnected and/or unmounted. Also note that there are power limits that some external drives exceed on the USB/Pi4 bus. External power to the drives can help with this.
Let us know what you find in the system logs. You will likely have to use:
with it’s variety of switches to look at the journal. We have work to do on our system journal interface that will likely arrive in time.