preupgrade, F15 -> F16 anaconda hangs.

[linux guy]

I'm upgrading my XPS17 laptop from F15 x64 KDE to F15 via preupgrade.

Preupgrade ran fine.

It reboots fine.

After a bit, I get a blue screen with "Running anaconda 16.25, ... -
please wait." at the bottom.

ctrl alt F2 give me a command line in a new session.

"top" shows 0%s all the way down for CPU and memory, except for a blip
for systemd once in a while.

I don't see any obvious problems using dmesg.   I do have a few "md:
raid personality registered for level X" messages, which surprises me.

The laptop is equipped with a single 160 GB SSD drive.  It is NOT dual
boot, it is Linux only.

How do I fix this problem ?