Suggest the typical hardware device troubleshooting. watch/tail your dmesg -w or kernel log as you add the extra drive. It’s curious that the system itself doesn’t crash, but from your description it still sounds like a power starvation concern or possibly high temperatures if this device is under heavy load.
Problems with spinning HDDs on RPI are common. Possibly even on powered hubs they may spike more than rpis 1.2A USB max allowance. You may need a specialised hat, or better to switch to solid state storage?