Adding a hard drive

Connect the external hard drive to your iMac (is it a USB or Thunderbolt type external disk?). Then run Disk Utility (located in /Applications/Utilities) and then set view option to "Show All Devices" and then select the device name (name should be obvious but if not do a screen capture of what you see and post in reply and will give more feedback).

Then use Disk Utility to format it as HFS+ using only one partition using the "Erase" tab (if you want more than one partition you need to use the Partition button not the Erase button), and create the "name" (e.g. ExternHD), specify "format" as "Mac OS Extended (Journaled)" and select "scheme" as "GUID Partition Map" then click "Erase" button -- note this will erase everything on the drive so if there is any special software that came supplied on the drive make a copy of it before erasing, in case you need it later.

Once it has been formatted the OS will mount it and then you can start using it. Where are the photos now located you wish to "move" to it? Are they already saved in a Photos library or ???

If you have any more questions ask.

Good luck...

The advice that others have given is accurate but I would just add a comment that the Apple link refers to copying the Photo library to your external hard drive. This means, I think, that you can read the library only via a Mac. To save the photos as separate .gif or .jpeg files that can be read by just about any device, select the photos or album in the Photos app and then use the “Export” option to save them in their original format on the external hard drive.

Just to add to dot.com's great advice...

https://support.apple.com/en-us/HT201517

Thanks! :)

Great to hear Dolly! :)