What is the problem with boot selection on Imac 27" 2011?

Question

iMac

There is an Imac 27 "2011 with Yosemite
When creating a bootable USB flash drive, after rebooting and pressing the key, it gives boot options, there is only its own system disk and recovery, it does not see an unconnected hard drive via USB or a flash drive.
I tried different flash drives, the same flash drive on another one for sure the same poppy looks
great.Flash drives and drives after booting into OSX are also visible and read perfectly, just as visible in Disk Utility.Just
a few days ago everything was fine and everything was
visible.After that, OSX was upgraded to Yosemite and the bootcamp with Windows was installed 8.1, then 8.1 was successfully demolished.