There are a couple things to try. You can try to format it using the HP USB format tool, you could try disconnecting the main HDD and running the XP install disk and formatting it (works with stuborn RAID drives that wont want to format when setting them back to non-RAID drives) or you can try the Gparted disk.
Note with Gparted you need to select "Advanced Mode" and choose the "Versa" video drivers to get the disk to work.
Note with Gparted you need to select "Advanced Mode" and choose the "Versa" video drivers to get the disk to work.