fdisk/disklabel - Error: unable to write data to disk ad0

Hei!

On Thursday May 29th I installed FreeBSD 5.1 Beta2. The next day I've
update to CURRENT from May 30th. I have a 60 GB ATA harddisk and during
installation I created only 1 20 GB slice for FreeBSD. Today I wanted
to use the other 40 GB of my hard disk and create two more slices each
20 GB. Well, fdisk reports that the disk geometry is wrong and sets it
to 7476/255/63, the same disk geometry the BIOS reports. When I try to
save the changes in fdisk, I get the following error messages:

      Error: Unable to write data to disk ad0
      Disk partition write returned an error status!

Hmm, I tried both in multiuser and singleuser mode.
Finally I booted with the floppies I had used to setup FreeBSD 5.1B2.
Fdisk also reports that the disk geometry is wrong and sets it to
7476/255/63. But creating the slices and saving the changes did work:

The data for partition 1 is:
sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
----[snip]----
The data for partition 2 is:
sysid 165 (0xa5),(FreeBSD/NetBSD/386BS
----[snip]----
The data for partition 3 is:
sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
----[snip]----
The data for partition 4 is:
<UNUSED>

Now I have booted FreeBSD from hard disk and try to create partitions
within the new slices. But again I get:

    Error: unable to write data to disk ad0

Now I am booting with the install floppies again. I guess this is 
working fine.
Any ideas what I am doing wrong?

Regards,
Herbert