Playback from an external hard drive.

Hello Tom Cray,

Welcome to Apple Support Communities. If we understand correctly, you are setting up or thinking of setting up an external drive for your Mac.

You can put your files like photos, iMovie library or TV files on your external drive and play them from there. You will just need to tell your apps where to look for those files by designating a library. For instance, if you wanted to save space by keeping your photos on your external drive, use these steps from Move your Photos library to save space on your Mac.

"Move Photos Library to the storage device

1. Quit Photos.
2. In the Finder, open the folder that contains Photos Library. By default, Photos Library is stored in the Pictures folder of your home folder. You can open your home folder by choosing Go > Home from the Finder menu bar.
3. Drag Photos Library to a location on your external storage device. Learn what to do if you get an error.
4. After the move is finished, double-click Photos Library in its new location to open it.
5. If you use iCloud Photos, designate this library as the System Photo Library.

If you want to delete the original Photos Library to save storage space on your Mac, you can drag it from its original location (the Pictures folder of your home folder) to the Trash. Then empty the Trash."

If you move your iMovie library to an external device, you'd take these steps from Move your iMovie for Mac library.

"Move a library to an external storage device

1. Quit iMovie.
2. Select your external storage device in the Finder, then choose File > Get Info.
3. If the information in Sharing & Permissions isn’t visible, click the triangle , then make sure the “Ignore ownership on this volume” checkbox is selected. If it's not selected, click the lock , enter an administrator name and password, then select the checkbox.
4. Click your desktop to make sure that you’re in the Finder, then choose Go > Home from the menu bar at the top of your screen.
5. Open the Movies folder, then drag the file named iMovie Library to your external storage device.
6. When the library file is finished copying, double-click the library file on the external storage device to open it in iMovie. If an alert appears asking you to give iMovie access to a removable volume, click OK.

If you clicked Don't Allow, in the Finder choose Apple menu  > System Preferences, click Security and Privacy, then click Privacy. Click Files and Folders in the sidebar, then select "Removable Volumes" under iMovie.

7. Make sure the library you moved contains all your media, then drag the original file from the Movies folder on your Mac to the Trash."

You may have noticed in the iMovie article above that it says "You can’t store your library on a storage device used for Time Machine backups." In this case, if you use your external drive for Time Machine backups, you won't be able to use it for this purpose.

If you use Final Cut Pro, you might also find Move a Final Cut Pro library helpful. Also, check out Back up your Mac with Time Machine if you have not been using Time Machine since you mentioned backups as well.

Best of luck.