You should be able to turn off the mirroring, use a utility like Partition Magic or True Image to create the second partition, and strat the RAID mirroring from scratch.
The following is an excerpt from this article
Hardware RAID solutions work by creating the illusion that your two physical drives are a single unit. The array is managed by the hardware controller. You need software drivers in whatever operating systems you plan to use so that the operating system can work with the RAID array you create. As far as Windows is concerned, you have a single drive; the hardware hides the individual members of the array completely. (The same is true for other operating systems.) After you load the correct driver, you can partition the drives however you want.
The first step, of course, is to make sure you've got a really great backup, just in case something goes wrong.
Your hardware controller should allow you to break the RAID mirroring. Doing so should leave the data on the first drive intact. If so, then you're good to go. Re-partition that disk as you please, using Partition Magic to shrink the main partition and create new ones from free space. Then create a new array. If you're using a Highpoint controller, choose the Duplication option to copy the data on the source disk to the mirror disk. The hardware RAID controller doesn't care what's on the disk; its job is to mirror the content from one disk to the other for reliability's sake, and it works at a very low level, independent of operating systems. All of your partition information should be intact. When you view it from an operating system that has the correct drivers, it looks like a single disk, which you can manage using whatever partitioning tools you like.
Edited by dc3, 03 July 2008 - 09:53 AM.