Possible to run Photo from an external hard drive

Yes you can have your Photos library on an external drive provided that drive is formatted Mac OS extended (journaled), is on a local fast wired connection, is not a TimeMachine backup volume and is always available prior to launching Photos

No you do not have to switch to it each time you launch Photos - Photos always opens the last used library if it is available

Yes TM wil backup an EHD - in the TM preferences remove the EHD from the excluded list

Yes PS will save to that library - it is your library and everything works exactly the same as if it were on your internal drive - absolutely no differences of any sort

Yes iTunes can use an external drive but for details ask in the iTunes forum

LN

You can switch between iTunes libraries, and your iTunes Library can be on an external drive.

See the iTunes Help:

Use multiple iTunes libraries

You can have more than one iTunes library. For example, you could have a library of holiday music that wouldn’t appear in iTunes the rest of the year. Or you could keep your music in a library on your computer and your movies in a library on an external hard disk.

Create or use a different iTunes library

1. If iTunes is open, quit it.
2. Hold down the Option key while you open iTunes.
3. In the window that appears, do one of the following:
   • Create a new library: Click Create Library.
   • Choose a different library: Click Choose Library.

You can name your iTunes library (the folder containing your iTunes content and the iTunes Library file) whatever you like. For best results, if you create additional iTunes libraries, don’t name the new libraries “iTunes” or “iTunes Music.” Make sure each iTunes library has its own unique name so you’ll know which is which.

In each library you create, you can select different preferences for the iTunes Media folder, the types of content that are displayed, parental settings, and the library name. To set preferences for a library, choose iTunes > Preferences while using the library.

SEE ALSO

Apple Support article: What are the iTunes library files?

And Larry is right - ask about iTunes in the iTunes forum. There you can get advice how to consoidte the library before moving it to avoid dead tracks.

No - I Told you the requirements - only you can check that your drive meets them - in the finder get info for the drive and look at the information

you can have your Photos library on an external drive provided that drive is formatted Mac OS extended (journaled), is on a local fast wired connection, is not a TimeMachine backup volume and is always available prior to launching Photos

LN

I got that far- different guy here- BUT- when I make changes to imported files I can always revert to original, even if I've saved the changes; when I backup to and open them from external, that option, next to the done bar after clicking edit in Photo, disappears; so the backup to external is only saving my edited version, not whatever (computer-speak) data was in the original that got back to the original, is that correct? So far, everything I've backed up is still in my macbook but I want to clear room, so should I back up the edited and reverted to original files or is there a better way? Thanks, J