well, lets start with which openmanage cd from dell you are using, for the R420 you should be using this disc www.dell.com/.../poweredge-r720 you could also try using the lifecycle controller (F10 at bootup) and use the OS Deployment function. IF SLES 11 SP2 is not a listed option, you should then update the OS drver pack using the Lifecycle controller Firmware Update option to update the driver pack and anything else that it shows could be updated. This assumes that the system can reach ftp.dell.com. Please let us know how this goes. 
The native installer for SuSE Linux Enterprise does not see any hard drive, so I am trying the Openmanage cd from Dell. I am using the DVD ROM built into the server. When I am prompted to insert the "SLES 11 SP2 x86_64 DVD" I do so, and then get the error "Invalid media". I only find one reference to this error in the release notes, referring to mounted via DRAC or iDRAC.  There is no terminal available to dismount the media and then re-mount it as instructed in the note. I only configure the DRAC in the destination LAN when I deliver the hardware. Am I going to have to configure and use the DRAC, and if so, are we sure the the option to dismount and remount the media appears there, even though the media is not mounted via the DRAC? IMHO, the evidence of Dell  "support" for this operating system is tenuous.
SLES 11 SP2 on PE R420