P55 vs. P58 Raid Limitations??

You can still use single disks even in RAID mode, just create an array consisting of only one disk, as JBOD or RAID0. It would require using the RAID-drivers during installation, although Windows 7 appears to have some RAID-drivers already included in the installation medium.

This would allow you to set RAID mode on your onboard SATA connectors, and use both a single disk as C: system drive, as well as a RAID1 which will be D:.

As a related note, i would like to add that a RAID1 is largely unnecessary for consumers. While RAID1 gives you two "copies" of data on each disk, its mainly used to guarantee access to the data for high-profile servers. For consumers, its not a disaster if you don't have access to your files for just 5 minutes; you won' t loose 3 million per minute having no access to your data.

Instead, a backup provides a much greater protection for consumers, as it protects against much more dangers than RAID ever can. Think about accidental deletions, virusses, filesystem corruption, etc. All these risks will still corrupt your data on a RAID1 even with both disks being perfectly fine.

Using those both disks as backup solution instead would also solve your problem with RAID; as you wouldn't need RAID at all and just have three drives:
1) system drive
2) data drive
3) backup of data drive

That simple.