I have 2 drives that were mirrored on a desktop motherboard using the onboard raid controller/software (I'm pretty sure it is software RAID). I've moved them to a new machine and they are no longer RAID'd. I can't install new OS's on these now as I keep getting conflicts with the remaining raid partitions or data that is somewhere on the drive.

When I do an fdisk -l I get /dev/mapper/pdc_XXXXXXX at the end of the screen
I tried deleting the MBR of the drives and that didn't do anything.

How do I delete these drives?

Does zero'ing the drives work? something like: dd if=/dev/zero of=/dev/sda bs=1M? This would take forever with 2TB & 4TB drives so I'm wondering if there is a faster method that targets just the area that needs fixing.

It probably wasn't "software" RAID so much as "fake" RAID in that the controller was involved on old system. "software" RAID doesn't use a controller. Some controllers however are doing "fake" RAID with "software".

Anyway it appears the link below may describe the situation you're facing:
http://osdir.com/ml/linux.kernel.dev.../msg00014.html

That looks pretty close to what I'm experiencing. It sure was a real nightmare to deal with after the first install failure! This makes me think 2x about using any of these low cost onboard hardware MOBO raid solutions.

I'll try this out and report back