No need for an external drive, just keep the files in a separate folder, away from the application's folder. Poser has the tendency to use a Runtime dir inside the applications dir. Don't do that. 
I have series of dirs under ~/DSContent  that is shared both by DS and Poser. 

For example:
~/DSContent/Morphs
~/DSContent/Clothing
~/DSContent/Textures

and so on. 
In DS use the configure option and add each directory to the the Poser directory list. Just point to the same dirs in Poser as well. Keeping the content separate from the application makes it easy to update the application without losing your content. You don't need a separate drive, just a separate dir.

Paolo

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