How can I access my library from multiple computers? Can I store my Zotero library and associated files on an external drive?
The best way to access your Zotero library from multiple computers is to sync your libraries automatically via the Zotero servers.
If, for whatever reason, you cannot use Zotero's built-in sync features, you can also store your Zotero data directory on an external drive and use this to move your Zotero data between computers. You can set your Zotero installation on each computer to point to the external drive in the Advanced pane of Zotero preferences.
Another option is to run a copy of Zotero directly from a portable drive.