Facing trouble when trying to boot 5.2 with GRUB boot loader

[Klaus Juergen Osswald]

Hi,

I installed FreeBSD 5.2 with / on ad0s1a

Since I have other (LInux) OSes on the same disk,
I'm using GRUB booting FreeBSD
with

root (hd0,a)
kernel /boot/loader
boot

Boot doesn't succeed with message

Filesystem type unknown, partition type 0xa5
Error 17: Cannot mount selected partition

I tried another installation with
/ on ad0s3a & /usr on ad0s1f
booting with

root (hd0,2,a)

but getting same error messages at GRUB boot

Is there a problem to boot the new ufs2 filesystem with GRUB ?
How can I savely boot FreeBSD & my other OSSes ?