If you connect an external storage device and HFS+ partition doesn’t mount automatically, please make sure it’s formatted in a proper way.
Here’s an instruction how to create a partition in Mac OS X for Windows.
If it’s formatted properly but still doesn’t appear in Computer, open Disk Management -> right click on your HFS+ partition -> change drive letter and paths -> add a letter to your HFS+ partition.
First, try adding letter to your partition as described above.
If that option isn’t available, make sure your Mac OS X system partition is not encrypted by FileVault or any other way.
Also, check if you have Core Storage technology enabled in your configuration. First part of this article describes what it is and how to check it: CoreStorage support in Camptune.
This technology cannot be supported by HFS+ for Windows as it prevents Mac OS X system partition from being modified from outside when Mac OS X is not working.