Not booting after update failure

[Ian Price]

I have a Mac Model a1311 that had an update 2 days ago, It now will not boot, gets about 75% of the startup progress indicator, then displays "Install OS X"

If then I follow the reinstall procedure, it again fails after 75%

In the disk utility I can see all the apps, pics and docs etc, so do not want to loose them if I need to reinstall the OS from scratch, so If i need to can I back these up to a USB Device?

 

[honestone]

First, what OS are you using?

Secondly, was it the OS you were updating? If it was, what OS were you upgrading from?

Third, have you been making any backups to an external device?