2nd hdd migrating

I have a box with two disk spindles. One holds `/`, `/tmp`, and `/usr`, and the other holds `/var`. Both spindles are beginning to fail, so I want to migrate them to a new disk. I have installed a base system on the new (much larger) spindle, and would now like to copy the system from the old...

I need a hint how to mount a second hard drive into a newly installed BSD Desktop System. This second drive has been mounted under Debian wheezy but after changing to jessie I am facing the music. It is not a raid, justs a simple second hdd. Since I installed the new system and no further...