As Ed says, you need a backup copy of your library, however if you have the library files on the external drive as well as the media then the you don't have to worry about keeping multiple sets of library files up to date. Here is how I do it.
*Sync iPod/iPad/iPhone with two computers*
Although it isn't possible to sync an Apple device with two different libraries it is possible to sync with the same
logical library from multiple computers. Each library has an internal ID and when iTunes connects to your iPod/iPad/iPhone it compares the local ID with the one the device normally syncs with. If they are the same you can go ahead and sync...
I have my library cloned to a small 1Tb USB drive which I can take between home & work. At either location I use
SyncToy to update the local copy with the external drive. Mac users should be able to find similar tools. I can open either of the local libraries or the one on the external drive and update the media content of my iPhone. The slight exception is
Photos which normally connects to a specific folder on a specific machine, although that can easily be remapped to the current library if you create a "Photos" folder inside the iTunes Media folder so that syncing the iTunes folders keeps this up to date as well. I periodically sweep my library for new files & orphans with
iTunes Folder Watch just in case I make changes at one location but then overwrite the library with a newer copy from the other. Again Mac users should be able to find similar tools.
As long as your media is organised within an *iTunes Music* or *iTunes Media* folder, in turn held inside the main
iTunes folder that has your library files (whether or not you let iTunes keep the media folder organised) each library can access items at the same relative path from the library folder so the library can be at different drives/paths on different machines. This solution ensures I always have adequate backups of my library and I can update my devices whenever I can connect to the same build of iTunes.
tt2