When drive #2 - the "CDROM mounted in a cheapo external USB enclosure" mentioned in the original report - is used on the machine profiled in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=498834 the same thing happens: the Etch installer runs fine, the Lenny installer has much trouble. I don't have any other machines that can boot from USB on which this can be tested.