In DIM, go to the Advanced Settings tab, and at the bottom left side, there is a + sign button, where you add your content path for Poser or any other runtime/content folders you have on your external drive.  Click on the Accept button.  On the app's primary page, make sure that the Show Details feature is checked in the lower right-hand corner.  Now, when you select a content file for installation, proceed to the lower half of the page and change the Content Path Shortcuts field to the runtime of your choice.  Start your queue. This has always worked for me and I have similar arrangements on 2 computers with Poser and DazStudio.