usb flashdrive will not boot on mac
I have a Retina 5K Imac circa 214 (system report says "15,1"). Trying to boot from an USB flash drive formatted as MacOS extended (journaled) and GUID Partition Map. Ran "disk utility" on top-level USB disk , it showed bootable=NO. However, disk utility shows "bootable=YES" if I run it on the partition on the USB disk. If I reboot and hold down the Option key I do not see the USB flash drive. I also don't see it on using System Preference's Startup Disk item. Ran "First Aid" from disk utility on both he top-level USB disk and the partition and it does not show anything wrong. I have "Install MacOs Monterey.app" on the USB flash drive. I am running Big Sur on the iMac. Any suggestions?
iMac Line (2012 and Later)