How do I transfer photos from a 2015 iMac to an external hard drive?

The 27 inch iMac is a 2015 model. The external hard drive is a western digital 2 TB.

Is there any geniuses out there to help an old man out?

just in case this violates anything Apple wants to do like charge you $14,000 a month on top of paying for complete system from Watch to phone to computers to laptops which I don’t think it does and I don’t think that’s what these very nice tech people would do.

Sincerely, a customer that would like to remain one

[Edited by Moderator]