I ran into some issues when installing 6.1 on a machine with a DAC960-based RAID array. Using Disk Druid I created a number of partitions on /dev/rd/c0d0 (/, /boot, /var, /usr, /usr/local, /home and a swap partition). Disk Druid made one primary partition, put /boot on it, and put the rest inside an extended partition starting at c0d0p5. More specifically, / was on c0d0p9. Unfortunately the minor device allocations for the DAC960 drivers only allow seven partitions per virtual disk, so / could not even be mounted and the install failed with an error about not having space on / (as it was trying to do the install into the ramdisk). Running in expert mode, using fdisk to avoid wasting two primary partitions and paring down the list by one was enough to get everything to fit. It would be nice if Disk Driud didn't waste those primary partitions. It would also be nice if the DAC960 driver didn't have such a tiny partition limit, but that's not something I expect you can fix.
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We've made some changes to disk druid to use primary partitions slightly differently than in the past, but it does not know about the limitations of the RAID array you have. I would recommend using fdisk since it allows you inifinte flexibility (at the expense of user friendliness admittedly).