If you have your /home on an external HDD, no need to mount it during installation. You can proceed with installation and after installation is completed, can edit /etc/fstab to mount your /home on external HDD.
While you may be able to specify your separate /home yet choose _not_ to format it in the installer, it is prone to mistakes. You will never know how you will be surprised ;-) There
was (is?) a bug with ubiquity (the Ubuntu/Mint installer) that if you chose, "Replace OS with.." option, it will not just format the / partition of the previous release, but the whole drive! It's fixed in Mint (for later releases of Mint 17) but still not in Ubuntu if I've followed correctly. Ofcourse, it may not affect you as you have your /home (i.e., imp data) on a separate disk, but serves as an example.
Oh, another thing; I think ubuntu installer does not recognise the previous encrypted drive unlike anaconda (the Fedora/CentOS installer). So, if you want to reuse your previous encrypted drive, you have to first decrypt and use that. Also, before you reboot, you need to update /etc/crypttab otherwise, it will not boot up.