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.