If you're using Poser 5 or 6 you can simply link to that runtime on the external harddisk. Poser doesn't care WHERE it is, as long as it knows where to look :o) You could simply copy the whole existing runtime over if you wanted and empty whatever was on the main harddisk.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.