This is now solved.  I entered BIOS settings from the console, and configured the SSD as a Non-RAID drive, and made it bootable. The OS installation now sees it, and has installed to it.  
Hi We've just purchased a PowerEdge T640 with the following disks: 1 x 1Tb SSD 5 x 1.2Tb HDD   The 5 1.2Tb drives have been configured in the Bios as a RAID set, and a Virtual Disk has been assigned.   I now need to install Linux on the single SSD, and am having difficulty in actually seeing it during deployment!  Booting from a USB stick to install the OS only shows me the RAID disks to install to, not the SSD. Have I missed out something blindingly obvious?  How can I get the single drive to be seen? I've got iDRAC working OK, and booting from PXE / USB / Virtual Image is no issue, just the lack of disk!   Thanks in advance Dave  
PowerEdge T640 want to use SSD for O/S instead of RAID disks