Just installed my first Linux, Ubuntu 8.10. When the live cd insisted it had to install in the C: drive, I removed both C: My XP sys, and D: a data disk. and loaded Ubuntu

onto F:
No problem on install or start up. Looks good.

When I put the original XP & Data disks back, I used a boot manager called GAG to set up loading.
It found the install in its set up, but when I try to run the ubuntu, I get a error Sector Boot Not Found or Invalid. The XP works, and the second time I tried to select the

Ubuntu, It booted XP instead. If I remove the XP disk, the Ubuntu boots and runs,

When running the XP it cannot see the ubuntu disk. In the windows disk management screen, the ubuntu disk shows up as Disk 0, with 2 partitions, a healthy (active)

partition with no disk letter and a healthy (unknown partition) with no disk letter.

Looking for ideas on where to go from here.

You appear to have an incorrect entry in whichever boot file points to Ubuntu. Never used GAG but suggest you check their site for documentation or read what you might have. Windows will not recognize Linux partitions but third party software is available. Linux will recognize and read/write to windows.

If Ubuntu boots and runs when you remove the xp disk that means the appropriate files are in the root directory and the mbr of that drive. You don't have the correct settings in your GAG bootloader to point to Grub on the Ubuntu drive.

Unfortunately, the GAG boot loader has no settings. It's supposed to work with almost any operating system. Select drive, select partition, Type in a name - doesn't matter what the name is, it loads whatever it finds in that partition on that drive.

But all of the troubleshooting seems to assume you installed them on the same hard drive. How do I point to grub?

Do you mean with the GAG bootloader? Have no idea, never used it.
If I'm reading your post correctly, you have three physical drives and Ubuntu is on a drive by itself, correct? When you have only the Ubuntu drive connected, Ubuntu boots. When you have all drives connected, you can only boot xp.

Problem, GAG bootloader not configure right! You need to fix that or use the xp bootloader or Ubuntu Grub to boot both systems.

gag chainloads the other partitions - you need to install the Ubuntu grub to the root partition rather than the MBR of that (Ubuntu) disk.
Get the Ubuntu system up, and from a terminal try thisThen stick the other disks back in and see if gag will talk to it.