Can't transfer photos from old imac to usb drive!

Hello,

My family has an old imac (circa 2011) with a LOT of photos on it. I followed a tutorial online to format a usb drive and transfer my entire photo library onto it, but it just stayed "loading" at 0% progress for over 24 hours. Any suggestions or help would be greatly appreciated! The computer is very slow and glitchy, so we just want to get the photos off it then get rid of it. Thanks!