hi. I have fc3 on hda and ubuntu on hdb. FC3 has been on there for awhile and ubuntu is newer. When i installed ubuntu i reinstalled grub to the mbr. Now im going to take the drive with ubuntu out of the system and replace it with a bigger drive. Will grub automatically find the other conf file in FC3? Do i need to reinstall grub from the FC3 drive sot hat it will use that conf file? Can i change which config file it uses by dropping to the command line in grub at startup? Im worried that i'll make it difficult to boot up into FC3 if i just swap out the drive. Thanks for any replies in advance, i appreciate them.

I would make the grub change before even removing the drive. When you type grub, statement, root (hd0,0) tells grub to look in /dev/hda1 for the grub.conf file. The statement, setup (hd0) tells grub to load the mbr of /dev/hda ( your first drive )

For example:

"Will grub automatically find the other conf file in FC3?"
nope - loader code is very dumb. Not much smarts will fit in the space available in the MBR.
"Do i need to reinstall grub from the FC3 drive sot hat it will use that conf file?"
yep
"Can i change which config file it uses by dropping to the command line in grub at startup?"
yep - should work, although I haven't tried it with the initial install drive physically missing.
"Im worried that i'll make it difficult to boot up into FC3 if i just swap out the drive."
Well founded concern - see homeys post above.

right now it seems to me as if you have grub installed on the MBR of both drives. The only one that will work at this point is the one on the master hdd and it will use the grub.conf on the master hdd. It is this grub.conf that should be edited to include the other OS (hd1,0) probably

Not true.
The loader code is aware of which drive has the stage/conf files. It doesn't need to be the master - probably usually is, but not required.