What kernel are you running? You need at least 2.6.30, and preferrably 2.6.32.
If you have 2.6.30 or later, what do you see in dmesg | tail -50
after you try and fail to convert to RAID6??
front
.opinion
.Christian
.Projects
..Openmoko
..LinuxKernel
...NFSserver
...SoftRaid
....mdadm
...LaFS
Admin