Here's one way to do what you want:
Make sure the drive is formatted Mac OS Extended (Journaled)
1. Quit iPhoto
2. Copy the iPhoto Library from your Pictures Folder to the External Disk.
Now you have two full versions of the Library.
3. On the Internal library, trash the Events you don't want there
Now you have a full copy of the Library on the External and a smaller subset on the Internal
As a general rule: when deleting photos do them in batches of about 100 at a time. iPhoto can baulk at trashing large numbers at one go.
You can choose which Library to open: Hold down the option (or alt) key key and launch iPhoto. From the resulting menu select 'Choose Library'
You can keep the Library on the external updated with new imports using iPhoto Library Manager