Hi.

I asked something similar to this question when I first started using linux about my MBR.

I've googled but I can't find a single answer on the subject.

I have a USB hard disk set as my first boot device in the HDD list

My linux distro sees the HDD as /sdb2 as it's the second partition on the USB disk.

My question is if I upgrade to 9.04 or a later version would it update grub and possibly break my main SATA HDD's master boot record?

is sda1 the device with the MBR it puts grub onto? or is hardware 0 considered the first boot device in which case is that what my BIOS sees as my external HDD when I use linux?

This is something I am really concerned about as there are many custom features that came with my PC including a recovery partition that a boot up disc reads in case the windows system fails and I don't know if changing the MBR would effect it.

Also on the partition are expensive windows software which I don't want to lose.

9.04 or 10.04 ? Regardless, Ubuntu upgrades (as opposed to full re-installs) tend to be problematic in my experience.
The MBR exists on the device (/dev/sda or /dev/sdb) not any partition. Seems like you want Ununtu to install grub in the MBR of the USB and leave the internal alone so Windows boots natively if you remove the USB.
All that can be achieved, but you have to be a little careful to get what you want. Not the end of the world if the internal MBR gets overlaid - data in partitions is unaffected, and the Windows MBR is easily recoverable if you have the CD/DVD.

I know you don't install grub on a partition I need it to install on the USB device the distro is on but I need to know does that device class as hd0 in the BIOS?

And yes I have the windows reinstall discs and also a system recovery disc.

Hd0 is not a name that belongs to a certain drive.
Hd0 is the first boot HD in the boot priority .
Meaning if you change the boot priority of the HD a other HD become Hd0

That's what I thought.

Thanks.

Just wanted confirmation of that to be sure I don't mess up the windows bootloader.