THIS ONLY WORKS FOR PEOPLE LIKE ME WHO HAVE MILLIONS OF EXTERNAL RUNTIMES AND SEVERAL REMOVABLE EXTERNAL DRIVES, AND WHO CANNOT BE BOTHERED TO ADD THEM TO THE POSER RUNTIME LIBRARY BY HAND ONE BY ONE ALL OVER AGAIN.

My humble tip is probably more useful for non-techie artists using multiple runtimes on multiple removable external hard disk drives.

Once you've got your external runtimes set up inside Poser's library ( I can only speak for Poser 6 and 7 inside a Windows XPSR2 environment), you have to be aware that Poser also notes down the drive letter of the external removable storage.

( It does this through writing the libraryprefs.xml file in your user profile inside Windows etc etc, but never mind about that until later in this post).

Therefore, if you remove those external drives, or reboot and re-connect them in a different order, your computer's operating system may assign a different drive letter to the drives from the one they had when you added them to the Poser library. 

Which gives your good old Poser a big fat headache, not tonight darling user, hangs, dies, etc etc.

PART A
You can change your drive letter to the orginals by going into your windows Help and Support link from clicking the Start menu.  Ask for "Change drive letter" in the blank, get the results, and follow those instructions.

PART B
Needless to say, at this point, you must know which were the drive letters for which of your removal drives, in order to switch them back to what Poser understands.
You can find this by:
1.  making sure you have administrator rights on your machine - if it's your own and it's been set up for you to use, most likely you got Administrator status.  If not, ask the friendly techie you would normally call for such things.

2.  Open a window to show the files on your hard drive.  Click Tools, then click Folder Options.  Then hit the View tab of the pop-up, and scroll down the options until you get Hidden Files And Folders.  Select to show them.  Click OK.

3.  Look for a folder named Documents And Settings, and go inside.  Can you recognise your user name ?  That's your personal setttings folder.  Or it should be All Users if no one else uses your computer, or you all share the same machine, whatever !!

4.  Get inside that one folder of yours, and look for Application Data.  Get inside that one.

5.  Look for the folder there called Poser ( could be "Poser 7" ).  Get inside and viola !  There's your libraryprefs.xml file.  Make a backup copy of that file first.  Then, open it with Notepad or Metapad.

6. You will see from the file paths of your multiple runtime libraries recorded inside the file,  the  drive letters assigned to removable drives for every folder that you have put into the Poser library. 

With a bit of detective work,  you can slowly start to identify the drive letters for the hardware configuration before it all got jumbled. 

Make a note on a piece of real paper which drives had what drive letter ( you can tell from the folders mentioned).  Don't edit the contents of this XML file !!!  Close without saving, after you have got the information you need.

7.  Flip back to Part A, and change the drive letters through your Computer Management utility (again must have Administrator's rights to your Operating System !)

PART C
Now that you have made Poser all happy again with where to find whatever folder and file, it should behave properly, and not lose your runtimes or hang.

Use my tips at your own risk.  I can't say this method will fix all your Poser-related runtime problems.  I can only say that this method has helped me manage my anarchic, chaotic runtime situation.

Eternal Hobbyist