download photos from iMac to external drive

Do you mean the Photos library, or a bunch of images in your own folder structure?

For the first:

Move your Photos library to save space on your Mac - Apple Support

For the second, just drag them over and be prepared to wait. If it is a laptop, make sure it is fully charged or plugged in.

If you want to copy some photos to the external drive for sharing?

You can simple drag and drop them from the Photos App to the external drive.

If you want to move your entire Photos Library to the external drive?

see > Move your Photos library to save space on your Mac - Apple Support

Please keep in mind, that if you move your entire Photos Library to an external drive.

You should backup the Photos Library to a second external drive to help prevent loosing your Photos.