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You need to add your external disk to the catalogue, or even better, just add the particular folders on your disk that you would like to be catalogued (this makes Quicksilver's indexing lighter).

In the Catalogue section of the Quicksilver prefs, click the + button at the bottom and add the folder/disk that you want to be indexed. Once added, press the (i) button at the bottom to show the drawer and enter what depth of contents in that folder/disk that you want to search. Then click the little refresh button at the bottom (rescan source) and you're done. Now it will search that folder/disk.

You can also do all of this through the quicksilver bezel interface, but I don't know what plugins you have installed so I can't be arsed to explain the steps.
:p
 
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