You don't say which "MacBook" you have. OS X El Capitan will run on:
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Can I back it up to usb memory for this to happen
For what to happen? You can create a clone of your MacBook's hard drive on external media with something like SuperDuper and this can be cloned back to the Mac's internal drive. SuperDuper clones are bootable.