When I create soft links within my /home folder to my other hdd (sdb) they no longer work when I restart my system. Do I have to mount my hdd to say /mnt/sdb on boot?
Softlinks are like a post-it note in a bookcase, stating "I am not the file you are looking for. Instead get it from this bookcase". If you have two bookcase, place your post-it note in the first one and remove the second bookcase than that post-it note will not help you.
If you always want it to be available then you will have to make a copy.