Trying to remotely help a friend in the EU get her 2011 MacBook Air back to El Capitan. She is not technically inclined but very good at using computers and following directions. Unfortunately she lets her students use her computer and one of them decided to update to High Sierra, making it really slow. I already know how to re-download El Capitan from the App Store, so that is no problem. But I have no personal experience with High Sierra or APFS (my own Macs are on Sierra and El Capitan). What will happen when she runs the El Capitan installer on her APFS SSD? Will it allow her to reformat the SSD as HFS and do a clean install? Is this all very straightforward, or is something special involved? I'm assuming that it won't be possible to install El Capitain over High Sierra without completely wiping the SSD, so she will need to do a clean install, then use Migration Assistant to move a backup of her files to the MBA. So, if she does a Time Machine backup to an external USB hard drive under High Sierra, will this work properly? Or will there be an El Capitan issue because the Time Machine backup was created under High Sierra? Would Carbon Copy Cloner be a better way to backup of the High Sierra machine before reverting to El Capitan? Thanks in advance for any insights!