Again, it's not difficult, but you have damage in your Library. So a first start will be to try rebuild it.
Option 1
Back Up and try rebuild the library: hold down the command and option (or alt) keys while launching iPhoto. Use the resulting dialogue to rebuild. Choose to Repair Database. If that doesn't help, then try again, this time using Rebuild Database.
If that fails:
Option 2
Download iPhoto Library Manager and use its rebuild function. (In early versions of Library Manager it's the File -> Rebuild command. In later versions it's under the Library menu.)
This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.
Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.
Going forward:
I love taking photos and my laptop gets pretty full so each year I put them on a hard drive.
Note two things:
1. By taking them out of iPhoto you lose the non-destuctive processing feature. If you don't wan that you might question if iPhoto is the app for you.
2. Using copy and paste won't get high quality version, just the sharing previews, which will be medium quality and missing metadata.
I don't want to store them on iCloud, as it seems as if this is something that apple is leading us towards.
Most pole users are now Mobile users. People with Phones and iPads. They regard the Cloud as a feature and that's the demand Apple is responding to. But it's no compulsory on any App. You can simply just not use it.
Also, save yourself some bother and just put the whole Library on the external.
Regards
TD