I am fond of blanking the partition table of any disk I am about to use
that has extended/logical dos partitions on it and writing the blank table
to disk. This seems to prevent some problems later on in an install where
disk druid will not make a new primary partition that will cross into an
old extended partition's area, even though it's officially gone. With
Guinness (and Pinstripe before that - could have sworn I reported it then
but I couldn't find it) entering fdisk and hitting 'o' then 'w' has no
effect. All the old partitions are still there when you reenter fdisk. The
temporary workaround is to 'd'elete each partition in turn and write the
blank table back to the disk.