Not really advice, just information on what I do myself:

1. I keep my connected Runtimes where access to them is fastest, on a local HD. I have an iMac, so I cannot add a second local HD, but I would keep them on one if I could.
2. I save separate copies of each product I download on an external HD (you could also burn them to CD/DVD). I expand every product to an individual Runtime before installing and saving it, for two reasons: a) to make it easy to install again (just ditto the Runtime into my library), and b) so that I can check the contents of an individual product. But you can save the downloaded zip/sit files themselves if you prefer.
3. I take regular backups or copies of my Runtime parent folders to an external HD.
4. I do not install any content into the Poser program Runtime. I have all my Runtimes within my Users folder (Mac equivalent of My Docs), for two reasons: a) so that they are included in my regular backups, and b) so that when I eventually uninstall Poser 7, I can just delete /Applications/Poser 7/ (Mac equivalent of the Program Files folder) without having to worry about losing any content.
   That's all I can think of for now...
   Izi

PS: in general, a very good rule of thumb is: keep programs and data separate, both on locally-connected (fast-access) disks, and if possible on separate disks.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)