"An ISP 'caches' certain popular content by storing those files on its local server. When users click to access cached content — which will typically include items on the home page but also could include other content as well — it is accessible directly and quickly from the ISP's servers, and the user need not download the pages over the Internet."
Regardless of its duration, though, whenever a temporary copy of information is created, it will likely be a copy not just for computer purposes, but also for copyright purposes. A fortiori a temporary copy on a hard disk is a copy for copyright purposes because such storage typically does not become erased when the power is turned off. Much caching is in fact accomplished by storage on hard disks, and many caching technologies result in the creation of quite persistent copies.