You may need to just reinstall grub to the f12 partition
I'd backup your menu.lst on /dev/sdb2 Also make sure you /boot/grub directory has these files
-rw-r--r-- 1 root root 63 2010-03-11 12:33 device.map -rw-r--r-- 1 root root 14872 2010-03-11 12:33 e2fs_stage1_5 -rw-r--r-- 1 root root 14036 2010-03-11 12:33 fat_stage1_5 -rw-r--r-- 1 root root 13344 2010-03-11 12:33 ffs_stage1_5 -rw------- 1 root root 1021 2010-03-11 13:54 grub.conf -rw-r--r-- 1 root root 13356 2010-03-11 12:33 iso9660_stage1_5 -rw-r--r-- 1 root root 14928 2010-03-11 12:33 jfs_stage1_5 lrwxrwxrwx 1 root root 11 2010-03-11 12:33 menu.lst -> ./grub.conf -rw-r--r-- 1 root root 13480 2010-03-11 12:33 minix_stage1_5 -rw-r--r-- 1 root root 16128 2010-03-11 12:33 reiserfs_stage1_5 -rw-r--r-- 1 root root 17488 2009-10-01 12:08 splash.xpm.gz -rw-r--r-- 1 root root 512 2010-03-11 12:33 stage1 -rw-r--r-- 1 root root 125432 2010-03-14 14:54 stage2 -rw-r--r-- 1 root root 13628 2010-03-11 12:33 ufs2_stage1_5 -rw-r--r-- 1 root root 12932 2010-03-11 12:33 vstafs_stage1_5 -rw-r--r-- 1 root root 15664 2010-03-11 12:33 xfs_stage1_5
Amusing, I didn't realize that menu.lst was a symlink, neat!
Boot into fedora 10 on /dev/sdb1
su -c "grub"
at the grub prompt
device (hd0) /dev/sdb root (hd0,1) setup(hd0,1)
If this doesn't work then I dunno what's going on. If these aren't the right settings for your Drive layout, send back a full overview of that and I'll try edit the command sequence for you.
How this Helps -Will