How are you moving them? Exporting does not remove material from iPhoto.
Why not have two Libraries, one on your internal drive, one on the external?
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
Note that you are not making a backup. To have a backup you must have your original and one or more copies of it. You are archiving photos not backing the up. If yor archive device failes you will lose your photos. If you primary library failes yor will not be able to restore it but will have to start over with a new iPhoto library
To back up you must have a copy of the entire iPhoto library