How do I save photos from iphoto on another hard drive?

You need to make a decision

You can transfer the photos but then iPhoto can not use them and the files are new so have new dates (the photos keep the photo date as it is part of the EXIF/IPTC data within the photo - totally different from the file dates) -- this is done by selecting photos and exporting to a folder on the EHD - see Exporting From iPhoto

Or

You can transfer the iPhoto library and run it on an external drive - this requires a drive formatted Mac OS extended (journaled) connected with a fast wired connection like FireWire, ThunderBolt or USB that is not a TimeMachine backup volume and that is always available prior to launching iPhoto - to do this quit iPhoto and drag the iPhoto library to the EHD then hold down the option key while launching iPhoto and select the new location

What is it that you want to do?

LN