On several different platforms, with several different atapi cd drives,
using cd's that have correctly installed on other occasions, the install
hangs with the syslog messages reading

stat=0x51
error=0x30

and finally
Error: unit attention sense key=0x06
power on, reset, or hardware reset occurred asc=0x29 ascq=0x00

This happened most recently on a system that had previously installed from
the same cd, and which, when booted up, was able to mount and read this same
cd.

it occurs at the time the shell is started, and /sbin/loader is kicked off.