I wonder if there is an easy way to solve this.
Maybe my initital mistake was when I installed linux (Mandriva)
on a primary partition quite some time ago...

To make room for a new distro for testing I repartitioned,
with parted or diskdrake - I do not remember - and installed some
new distro on the newly created ext3 partition.

It looks like this now:
# fdisk -l
Disk /dev/sda: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x48d8907c

Device Boot Start End Blocks Id System
/dev/sda1 * 1 7296 58605088+ 7 HPFS/NTFS
/dev/sda2 7297 11868 36724590 7 HPFS/NTFS
/dev/sda3 11869 13194 10651095 5 Extended
/dev/sda4 13195 14593 11237467+ 83 Linux
/dev/sda5 11869 13063 9598774+ 83 Linux
/dev/sda6 13064 13194 1052226 82 Linux swap

Windows (on sda1) and Mandriva (on sda4) boot and run just fine.
I can mount sda6 and access its contents (from Mandriva).
However, grub does not me boot the Fedora on sda6:
# grub
Probing devices to guess BIOS drives. This may take a long time.
grub> root (hd0,4)
Filesystem type is ext2fs, partition type 0x83

grub> kernel /boo
Error 2: Bad file or directory type

Is there a way out w/o deleting partitions 4,5 and 6 and restarting from
scratch by installing in secondary partitions?

No you should not have to start over but I am a little confused, you say windows and mandriva boot fine. Do you boot windows and mandriva from the same grub menu.lst.

Yes Larry, both are booted from boot/grub/menu.lst on sda4.
The menu also holds an entry for Fedora, with


title fedora
kernel (hd0,4)/boot/vmlinuz-2.6.25-14.fc9.i686 BOOT_IMAGE=fedora root=/dev/sda5 vga=790
initrd (hd0,4)/boot/initrd-2.6.25-14.fc9.i686.img

but that one fails with the "Error 2" mentioned earlier.

By the way, I have also tried Mandriva 2009 on that partition,
with the same problem.
Oh, I think I have grub in the MBR.

Check out this solution and see if it does not fit your situation. It is your bios and not your grub installation, you will find the solution about halfway down the thread.

Hi Larry,

Sorry, I do not understand which *this solution* or halfway down which thread.
Never considered my BIOS: PhoenixBIOS on a Toshiba Satellite Pro, cannot find version info.

part of the problem might be fedora9's grub uses a 256 bit inode , the others use a 128 bit
by chance did you NOT install the fedora9 grub in sda5 /boot
also fedora by default will put grub in mbr sda1

-- OR and most likely --
the problem may be that grub dose not work on a lvm /boot must be a ext3 but all of sda5 is lvm
so repartition sda5 and 6 into
sda5 /boot ext3 100meg
sda6 swap ~ 2 to 3gig
sda7 / the rest of the disk ( no format if you want to use LVM) or ext3
personally i hate lvm and have not used it since fedora5

Thanks for the suggestions, but I have now admotted defeat and repartitioned with Parted Magic.
Removed primary sda4 and logical sda5/6, grown extended sda3 to the remaining space,
created new logical partitions within sda3 and re-installed.
Not understanding what exactly was the problem irks me, but I needed to move on.
And it works.