Automator actions, geotags, and more advanced tricks for Photos for OS X
Automator actions
Photos does have some basic scripting support, but it doesn't come with any Automator actions. Automator can be an extremely useful utility for Mac users who want to automate tasks--these days I'm using it to customize shell scripts and execute scripts from the Finder. Apple provides Automator actions for iPhoto and Aperture, but sadly not Photos!
Fortunately, the good people at Mac OS X Automation have come through with a package of Photos-related Automator actions. Their Automator action collection adds support for:
Unfortunately, Photos doesn't support editing images in an external editor, but using Automator you could simplify the exporting of an image, modifying it outside of Photos, and importing that edited version as a new media item inside Photos. That's not quite the same as editing an image in place, but it's better than nothing.
Ways to merge libraries
I've heard from a lot of people who want to merge multiple iPhoto libraries into a single Photos library. I feel your pain--iPhoto got so slow a few years back that I split my library in two, so I've got two separate libraries that both number more than 20,000 photos. So, how to put them back together
There are a few options. If you're committed to using iCloud Photo Library, you can pour all your libraries into it one by one, and in the end you'll have one big library. (If you've got a large library like mine, prepare to pay Apple for the privilege--$10 per month for 500GB of storage, or $20 per month for 1TB.)
Photos will only sync one library, designated as the System Photo Library, with iCloud. So to sync multiple libraries together, you need to upload them in turn. Once your first library has uploaded to iCloud--you can check in the iCloud tab in Photos' Preferences window to see if it's done syncing--launch Photos with the Option key held down and you'll be able to open a different photo library. Once that library's open, open the Preferences window and in the General tab, click the Use as System Photo Library button. Then switch to the iCloud tab and be sure iCloud Photo Library is turned on. (To save time, set Photos to Optimize Mac Storage--that way it won't attempt to download all of the photos you just uploaded from the other library.)
Keep repeating this approach until all your libraries are in iCloud. Then if you want, you can choose the Download Originals to this Mac preference and create a single library on your hard drive that contains all the images from all your libraries. (Or you can keep the setting to Optimize Mac Storage and put your faith in iCloud.)
If you're trying to merge libraries on different Macs, this is even easier--log all the Macs in to the same iCloud account and have them all sync their libraries. The end result will be a single, merged library.
If you're not riding the iCloud train, then you'll need some help. Fat Cat Software makes a helpful $30 utility called iPhoto Library Manager that lets you merge iPhoto libraries in advance of importing them into Photos.
If you've got a copy of Aperture, which supports importing of iPhoto libraries, you can use that to build a master library. In Aperture 3.3 or later, choose Switch to Library: Other/New, pick an iPhoto library, then choose File: Import: Library and keep importing new libraries until you're happy. Then you can import your newly created Aperture library directly into Photos.
Geotagging photos
If you use a camera that doesn't have GPS capabilities--my Canon Rebel T2i SLR certainly doesn't--you may have used iPhoto to add map data to the images you took with that device. Unfortunately, while Photos will display location data, it doesn't allow you to edit it.
If you want to add location data to all of your photos that aren't taken by an iPhone, you'll need to import the images directly onto your Mac. I recommend Image Capture, a utility that's made by Apple and appears on every Mac. Import all your photos to a folder, and then use a geotagging utility to add map data.
There are a bunch of different geotagging apps out there. Photo GeoTag, Geotag Photos, and HoudahGeo will do the job. Most of these utilities let you use an app on your iPhone to record your location while you're shooting, and then transfer that location to your pictures by syncing up the clock on your iPhone with the time stamp on your image files. It's pretty clever.
Once you've added map data to your photos, you can just import those files directly into Photos, and the app will understand and display their proper geotag information.
Upload issues
If you've felt like your Internet connection has grown sluggish since you started using Photos, it may not be your service provider! I've heard from many users (and seen myself) that iCloud Photo Library can be quite a bandwidth hog. Even when Photos is not running, your Mac will continue to upload photos to iCloud. If you're seeing Internet slowdowns, open Photos, open the Preferences window, click to the iCloud tab and click the Pause For One Day button.
If your Internet situation improves immediately, consider leaving your Mac on overnight and clicking the Resume button before you go to bed, letting Photos do its job overnight when nobody in your house is using the Internet.
How Photos saves you disk space
Apple was worried that people wouldn't upgrade to Photos if they had to essentially duplicate their iPhoto library while converting it to Photos. Do you have enough room on your hard drive to make a copy of your iPhoto library Most people probably don't.
So instead, Apple uses a clever technique called hard links to make your new Photos library. Essentially, the photos in your iPhoto library and your Photos library are using the same space on your drive. They're kind of like Finder aliases, but... different.
It's all complicated Unix nerdy stuff, but the short version is that Apple can migrate your library from iPhoto to Photos without taking up much extra space on your hard drive. All the media files belong equally to both libraries. If you delete your iPhoto library, the photos don't go anywhere, because they're still part of the Photos library. It's a weird concept, but the most important thing to remember is that you can migrate a 50 GB iPhoto library to Photos without needed 50 GB of free space on your hard drive.