You may need to download the raid software.
From the manual:http://support.dell.com/support/edocs/systems/xps400/om/WD741A02.pdf
"If your computer does not have a spare hard drive, and the computer has reported a degraded
RAID level 1 volume, you can manually rebuild the computer’s redundancy mirror to a new hard
drive by performing the following steps:
1 Click the Start button and point to Programs→ Intel(R) Application Accelerator→ Intel
Matrix Storage Manager to launch the Intel® Storage Utility.
2 Right-click the available hard drive to which you want to rebuild the RAID level 1 volume,
and click Rebuild to this Disk."