I had this issue which was previously fixed by editing the etc/fstab file and adding the entry UUID=DRIVE_UUID MOUNT_POINT FILESYSTEM ARGUMENTS 0 1. I did a clean install of OS and now the problem persists again. I have enabled Mount at startup from Disks.
Basically, the hard drive is inaccessible to use for downloading anything, until I go to Files and specifically click on the second hard drive there.