OFFTOPIC:
I use an external drive exclusively (in essence) for one of my macs. I actually access it over afp (to my primary computer) and keep the password in my keychain, so if I'm not logged in already it does that by itself. The only problems I get is when I access the database from two terminals at once.
BUT there's a catch to this: instead of telling itunes where to find the database, I tell it to look in the default location- where I've placed an alias to the directory on my other machine. I do this so that if my other machine is down it gives me a warning that the library can neither be accessed nor created in the default location (it won't overwrite the alias) rather than proceeding to create a new library in the default location.
In your case I'd first consolodate your itunes library on your current machine. Then, I'd setup my new imac, set a new library location (advanced tab of itunes preferences) somewhere on the External HD (if the HD is the same one from your MP, use a different folder than the one you were using). After that, I'd import the MP library to get the file permissions in my library right, and finally set my library location to the music folder on my boot disk. I'd make an alias to the NEW library (the one named "iTunes", that contains both the XML files and mp3's) on the Ext. drive, launch itunes, rip a CD, check to see that it's still only writing to the Ext. drive ------ and only then would I delete the original library.