OS X :: Can Make A Bootable External Hard Drive

I was installing a software update on my mother's Intel iMac. While installing, the iMac told me that there was a problem and to contact the software manufacturer. When I went to reboot, it would not. All I get is the universal sign for stop (circle with a 45 degree angle line through it). My mother has no backups whatsoever. She needs her information. I am in need of a safe way to retrieve her information.I have tried the basic help from Apple's site--nothing worked. A friend told me that I might be able to boot the computer from a bootable external hard drive and pull the information off that way.