What does "external-sd" describe?
Android devices generally use an SD card (usually the Micro-SD format) to increase storage available for data and, as of Android 2.2, applications.
For most devices the cards are removable. Specifications for those removable cards as well as many other technical details can be found at the Secure Digital Wikipedia article.
If your device's SDCard is non-removable, it is a so-called "Internal SDCard", and you should use the internal-sd tag instead. For a full overview of storage related tags, please see the storage tag wiki.
Quick SDCard troubleshooting guide
Flowchart SD Card Troubleshooting by Izzy (CC-BY-NC-SA)