When I try to install this update on 10.6.8, it dies during the installation

During the install, just after it says "moving files into place with shove" it then goes into failed state and has a 'ok' button to continue & reboot.

The /var/log/system.log shows:

Jul 7 21:57:06: --- last message repeated 3 times ---

Jul 7 21:57:06 James-Lindemans-MacBook-Pro [0x0-0x1d01d].com.apple.SoftwareUpdate[404]: Input/output error

Jul 7 21:57:07 James-Lindemans-MacBook-Pro Software Update[404]: REQ: moving files into place with shove

Jul 7 21:57:07 James-Lindemans-MacBook-Pro Software Update[404]: REQ: install failed

Jul 7 21:57:07 James-Lindemans-MacBook-Pro Software Update[404]: SWU: installing "Security Update 2013-003, 1.0"

Jul 7 21:57:07 James-Lindemans-MacBook-Pro Software Update[404]: SWU: installing "iTunes, 11.0.4"

Jul 7 21:57:25 James-Lindemans-MacBook-Pro [0x0-0x1d01d].com.apple.SoftwareUpdate[404]: /: no supported helper partitions to update.

Jul 7 21:57:25 James-Lindemans-MacBook-Pro kextd[10]: updated kernel boot caches

Jul 7 21:57:25 James-Lindemans-MacBook-Pro Software Update[404]: kextcache returned 0

Jul 7 21:57:25 James-Lindemans-MacBook-Pro Software Update[404]: Running /sbin/reboot

I have Mac OS 10.6.8 installed.