Your hard disk, known to Linux as /dev/sda1, is not automatically mounted during boot. When going there in Dolphin it is automatically mounted at a location known as /media/home2. To be able to access the drive before going there in Dolphin you need to make an entry in /etc/fstab telling Linux to automatically mount the drive at boot. Make a backup of that file, open it with root permissions and add a line to it to mount /dev/sda1. See https://help.ubuntu.com/community/Fstab
The line will look something like this:
Make sure your mountpoint exists. You may need to tweak the options.
UUID=01234567-89ab-cdef-0123-456789abcdef /my/mountpoint/ ext4 defaults 0 0
To get the UUID of your drive, use this command in a terminal:
ls -l /dev/disk/by-uuid/ | grep sda1