F16 preupgrade crash

[Zdenek Pytela]

	Dear all,
I am trying to upgrade F15 to F16 with preupgrade. The procedure ends up
with 
anaconda: No upgrade root was found

In program.log there is
INFO program: Running... /bin/mount -n -t ext4 -o ro /dev/sda1
INFO program: Running... /bin/umount /mnt/sysimage
INFO program: Running... udevadm settle --timeout=300
INFO program: Running... udevadm settle --timeout=300
INFO program: Running... /bin/mount -n -t ext4 -o ro /dev/sdb2
INFO program: Running... /bin/umount /mnt/sysimage
INFO program: Running... udevadm settle --timeout=300
INFO program: Running... udevadm settle --timeout=300
INFO program: Running... /bin/mount -n -t ext4 -o ro /dev/sdb3
INFO program: Running... /bin/umount /mnt/sysimage
INFO program: Running... udevadm settle --timeout=300
INFO program: Running... udevadm settle --timeout=300
INFO program: Running... /bin/mount -n -t ext4 -o ro /dev/loop1
ERR program: mount: /dev/loop1 already mounted or /mnt/sysimage busy

storage.log ends with
DEBUG storage:          PartitionDevice.setup: sdb3 ; status:
 True ; controllable: True ; orig: False ;
INFO storage: set SELinux context for mountpoint /mnt/sysimage
 to system_u:object_r:mnt_t:s0
DEBUG storage:          PartitionDevice.teardown: sdb3
 ; status: True ; controllable: True ;
DEBUG storage:             DiskDevice.teardown: sdb ; status:
 True ; controllable: True ;
DEBUG storage:               DiskLabel.teardown: device:
 /dev/sdb ; status: False ; type: disklabel ;
DEBUG storage:               DiskLabel.teardown: device:
 /dev/sdb ; status: False ; type: disklabel ;
DEBUG storage:          LoopDevice.setup: loop1 ; status: False
 ; controllable: False ; orig: False ;
INFO storage: set SELinux context for mountpoint /mnt/sysimage
 to system_u:object_r:mnt_t:s0
WARN storage: mount of loop1 as ext4 failed: mount failed: (32,
 'mount: /dev/loop1 already mounted or /mnt/sysimage busy')
DEBUG storage:          LoopDevice.teardown: loop1 ; status:
 False ; controllable: False ;

In the console, the system doesn't seem to have either /dev/loop* or
/mnt/sysimage mounted.

Repeated several times, the same result.
SElinux is enabled/enforcing, still in default configuration. I have 3 ext
partitions, root should be on sda1, sda is an ssd disk, sdb is hard disk,
both with mbr.

Is there any howto or release notes one should read before installing F16?
Thank you,

-- 

--Zdenek Pytela