f16->f17 wiht preupgrade failure

[shino]

i was trying to upgrade my f16 using the preupgrade procedure:

#yum update
#yum install preupgrade
#preupgrade

but unfortunately after a reboot i got an error:"unable to read package metadata".
looking through the logs i saw:

 INFO anaconda: Error downloading treeinfo: [Errno 14] Could not open/read file:///mnt/sysimage//var/cache/yum/preupgrade/treeinfo
 INFO anaconda: Error downloading treeinfo: [Errno 14] Could not open/read file:///mnt/sysimage//var/cache/yum/preupgrade/treeinfo
 WARN anaconda: Try 1/10 for file:///mnt/sysimage//var/cache/yum/preupgrade/repodata/repomd.xml failed: [Errno 14] Could not open/read file:///mnt/sysimage//var/cache/yum/preupgrade/repodata/repomd.xml
 WARN anaconda: Failed to get file:///mnt/sysimage//var/cache/yum/preupgrade/repodata/repomd.xml from mirror 1/1, or downloaded file is corrupt

and indeed there is no /mnt/sysimage//var/cache/yum/preupgrade directory despite the fact that it is there while i'm looking at the '/var/cache/yum/' dir from under f16 level. the problem was reported few times but as far as i understood it was related to the http installation (not the case), LUKS protection (not the case), multiple fedora installation (not the case), /var being on different partition/drive (not the case). what else can i do to be able to use f17 without of course need to install it from scratch? let me know if you would need additional informations.