I had a similar problem. I fixed mine in following way,
Get the uuid of the external hard drive with following command
Add the entry into fstab so that when you restart the raspberry device it is auto mounted every time
sudo nano /etc/fstab
UUID=0A423D084xxxxxx /mnt/data ntfs defaults,noatime,auto 0 0
Some explanation for above command - I mounted it to /mnt/data directory. ( You need to create it first if doesn't exist with sudo mkdir /mnt/data ) Mine was a NTFS format. You might try only the defaults option. I added noatime and auto to experiment a few things. You can check more details about it on internet.
Some Raspberry device ( including mine ) needs one more addition ( optional, try only if above doesn't work ), after above command, add a 5 seconds delay to loading the device as sometimes the Raspberry OS loads faster than external drive and external hard drive loads a little slower and need more time to start, in this case the external drive isn't auto mounted and it is skipped with an error at the start. Add the following property/value to the end of your cmdline.txt inside /boot folder
sudo nano /boot/cmdline.txt
Hope it helps!