Hi all
I have a MacBook Pro running 10.5.8.
I have an external USB drive, partitioned, GUID.
I have cloned a Leopard install DVD to one partition 1 (8.5Gb).
I used that to install Leopard to Partition 2 (30Gb)
Booted from Partition 2 - boots fine.
Used Software Update to upgrade to 10.5.8 - update proceeded normally.
Partition 2 will no longer boot - grey screen with big apple at centre - daisy wheel spins forever.

Next time I installed to Partition 2 with the Leopard DVD.
Booted from partition 2.
Used a downloaded 10.5.8 combo updater.
Same result.

What gives?

What make and model external drive do you have? Proprietary firmware in some enclosures make them only marginally suitable or even unusable as a boot drive. If you have a Western Digital drive you might look at WD external hard drives that are Mac bootable and how to format them to install an operating system.

Thanks for your response.
Further experimentation has shown the enclosure to be the problem.
The question remains: why would 10.5.0 boot, but not 10.5.8

My guess the reason is subtle differences in the boot process between the two versions of OS X that trigger an incompatibility with something in the firmware. It may even be the result of a tightening of constraints or change to eliminate some problem identified with the previous version.

There are standards, although the USB standard has criticized for leaving too much out, and Apple appears to try very hard to adhere to the more stringent interpretation of those standards, but not manufacturers are as stringent as Apple. If peripheral manufacturers see an opportunity for some optimization or feature enhancement that might benefit the big market, Windows PCs, they may take liberties with, fudge, or even ignore the standards. On the other side of the coin PC manufacturers often build in a lot of tolerance for "semi-standard" devices at the expense of some loss of reliability and performance.

Boot in verbose mode (hold apple and V at chime until you see text)

Tell us what you see. We don't necessarily need to know it line for line, lines that repeat or the last few lines before it stops updating should be enough, or anything that looks like an error. (some error-like messages are "normal")