I’m running a Ubuntu server on my old laptop with an external HDD connected to it. The external HDD is powered independently from the laptop, as it is plugged into the wall.
During a power outage, my laptop remains operational due to its battery, but the HDD shuts down. When power is restored, my laptop does not automatically remount the HDD, and I have to reboot the system manually to access it.
Does anyone know how I can resolve this issue?


That risks breaking the drive eventually. They don’t handle loss of power well. And if its being written to at the time, data loss and corruption is inevitable.
That said, no need to reboot to mount the drive.
sudo mount -aI shall give this a try the next time it loses power. Thank you.
Then set up a cron job that periodically tries to read a file on the drive, and if that command fails, then execute
sudo mount -aLike, set up a root account cron job to run
touch \external_drive\test_file.txt || mount -aat 5-minute intervals.