I successfully installed a 32-bit copy of 16.04 on my old 2006 MacBook Core Duo from a USB stick with help from rEFInd. Now I've mucked around a bit and want to reinstall a fresh copy of Ubuntu. The problem is when I formatted my hard drive during the first Ubuntu install, it looks like I destroyed the boot manager, and definitely the rEFInd functionality. It does not boot into a USB drive now, either with the original USB stick I used or with a newly formatted USB stick made with the Ubuntu Startup Disk Creator.
Normal boot procedure now flashes a folder icon with a question mark for a half of a second before booting to the current Ubuntu. When the USB stick is inserted, holding down the Option key shows only one partition called "Windows", which is the Ubuntu volume. Any thoughts about how to regain the ability to boot from a USB device?