Based on observations of the failure mode of anaconda for an in-place 
upgrade to 6.4, I used a heuristic method and solved the problem.

I made a backup copy of /etc/fstab, and then edited fstab to remove all 
references to any physical hard drive (e.g., sdb, sdc) that did not 
contain the images that needed upgrading.  For upgrade, anaconda 
evidently depends upon the contents of fstab; this change made anaconda 
ignore the disks with which it found issue.  After the upgrade and 
reboot, I then copied the saved fstab over the edited fstab, issued the 
appropriate mount command, and everything seems to be working.

I presume that this will not work when we migrate to SL 7 and must only 
have the target hard drive physically connected.

Yasha Karant