If it's USB-C based, unmount the external drive, then pull the connector, wait a few seconds and plug it in again. I had a different instance yesterday once. see postI'm having an issue with this update and beta 1 with my external drive not connecting to my Mac via external dell display, anyone else having issues with this? I've tried different ports and updating the software but nothing seems to work. Bit of a pain cause my Time Machine backup is now getting quite out of date.