In many cases you might want to deploy Windows to a computer with an external disk rather than load the whole shebang over internet. Officially only USB flash drives are supported. You most probably have a large deployment and USB flash drives are not very fast, especially the ones with large capacity. If you really want to use an external disk as a deployment source there is a workaround.
Basically you have to make an installation media in MDT.
Make a new installation media. Make sure NOT to use the path to your external drive, but make sure you choose a partition that has enough space for the whole deployment. We will get to the external drive later…
Unless you are doing something specific the Rules and .ini-file for the image should be the same as for the main deployment-share. Make sure you have the same rules and bootstrap file as for your main deployment share. To check this just right-clik on the “MDT Deployment Share” and choose properties -> Rules to check what is in there. Copy the same info to the installation media.
Right-click on the media and choose “Update Media Content” to consolidate all the files and folders for the media.
Now you should have a folder with the litetouch ISO file. Use Rufus to make an installation flash disk of the ISO. Just make sure you choose “List USB Hard drives” in Rufus and select your external disk.