It always pays to read the instructions...
I've had my Arduino Galileo for a few days and wanted to get it to boot from the micro SD card so I could log in and get to the Linux command line via SSH.
I downloaded the Linux image and put it on the SD card as described in the Getting Started PDF, but it kept booting the built-in OS (which doesn't include SSH). The SD LED on the board would flicker a couple times briefly when powered on, but the board seemed to ignore the SD.
I "solved" the problem with booting from the SD card quickly and easily by upgrading the board's firmware using the Arduino IDE for Galileo, as described in section 6 on page 7 of the Galileo Getting Started PDF.