how do I move just some events from iPhotos to external hard drive?

Do you want to keep them in iPhoto?

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

Some Notes:

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

Or just move the Photos to the Finder:

File-> Export

This User Tip

https://discussions.apple.com/docs/DOC-4921

has details of the options in the Export dialogue.

Select the Events you want to move to the EHD, use the File ➙ Export ➙ File Export menu option and set it up like this:

Select the EHD as the destination for the export. This will give you a folder for each Even selected containing the original files. If you would rather have the edited versions select Kind = Current.