Is it possible to save photos to an external HD and also run iphoto/ photo on my laptop?

Sure - but what you want to do is not clear

You can archive photos off your drive and you can run with the entire library on an external drive in either iPhoto or Photos

And you can not "move" albums to Photos - you migrate your entire iPhoto library to Photos and all of you albums are migrated as they are

LN

Moments in Photos are the new Events, i.e. groupings of photos sorted by date taken.

When the iPhoto Library was first migrated to Photos there is a folder created in the sidebar titled iPhoto Events and all migrated iPhoto Events (which are now Moments) are represented by an album in that folder. To open the sidebar if it's not already open use the Option+Command+S key combination. Any albums in your iPhoto library will be reproduced in the sidebar of the Photos library.

There's a way to simulate events in Photos.

When new photos are imported into the Photos library go to the Last Import smart album, select all the photos and use the File ➙ New Album menu option or use the key combination Command+N. Name it as desired. It will appear just above the iPhoto Events folder where you can drag it into the iPhoto Events folder

When you click on the iPhoto Events folder you'll get a simulated iPhoto Events window.

The downside to the simulation is that the Albums/Events can only be sorted automatically by Title. But they can also be sorted manually, either in the sidebar or in the folder's window at the right.

Ask Apple for more sorting options in Photos via https://www.apple.com/feedback/photos.html.