Why do Bad Things happen after installing Unslung 4.20 beta?
What happened for me was that I was getting inconsistent behaviour after installing Unslung 4.20 beta, which now I am fairly sure was simply that the unslinging had worked, but I had failed to put the device out of maintenance mode after doing the upgrade to 4.20. This meant that everything I tried was filling up device memory, with confusing results (not always recognizing the disk after reboot, some packages install, others do not, etc). The key point was that after unslinging and rebooting I was still seeing this from df:
Whereas when it works I see this:
with a 160 GB disk, with some data already copied onto it.
(I had previously failed to setup a 1 GB USB flash disk as the primary disk, but now suspect that the problem was the same in both cases).
Eventually the slug refused to boot at all, but the RedBoot rescue method using HTTP worked fine for me, and from that point everything worked properly - presumably because this method of re-flashing does not leave the device in maintenance mode.
Addition: Some drives are not detected if they are set to automatic power on at boot time. (For example the Buffalo DriveStation can be set to Manual or Auto. To work with 4.20 it must be set to Manual) If you see the above behavior from df, check to make sure your drive is set to always-on/manual.