With USB, there are various methods that the hardware can be setup to be recognised by the system, and the bios will only boot certain kinds. Its like 10 years ago, to boot from a CDrom (common by todays standards) you needed the disk formatted properly, the drive to support it, and the bios setup to boot from optical media. If you didn't have all three stars aligned, it didn't work. SD card readers aren't designed or setup to allow booting, unless specifically made to do so (I'm not aware of any). Yes, you can install onto an SD card, but that doesn't imply that you will be able to use it.
USB flash drives are a viable way to do this however, you can google and read up on that.