At installation time ify ou see something like the following:
/dev/sdE0: M4-C5064M4SSD2 /dev/sdE0/data /dev/sdE0/plan9 term% inst/start ... The following Plan 9 disk partitions were found. /dev/sdE0/plan9 Plan 9 partition to subdivide (/dev/sdE0/plan9)[/dev/sdE0/plan9]: This is disk/prep; use it to subdivide the PLan 9 partition. If it is not yet subdivided, a sensible layout will be suggested; you can probably just type 'w' and then 'q'. there's a fat partition where the plan 9 partition table would go. If you really want to overwrite it, zero the second sector of the disk adn try again
% dd -if /dev/zero -of /dev/sdE0/data -count 50000
That will wipe out the first sectors of the disk and allow Plan 9 to continue installation.