How to update IDRAC6 firmware from DOS, boot USB. Reason: T310, because on start server IDRAC6 communication error.

Thanks for posting!  First, if you are seeing a message on POST about a communication failure with the iDRAC 6, or any other DRAC for that matter, the DRAC is not talking to the system board.  The first thing you need to do is get those two components communicating before any other work can be done.  Reseat the DRAC card, and keep it out of the system for at least a minute so the flea power will drain.  If you have an iDRAC 6 Enterprise on a 200-500 series server (i.e. T410, R210, T510...) then you will also need to reseat the iDRAC 6 Express card.

Once you have reseated the card, power the server up and hopefully the communication failure error is gone.  If it is still present, either the DRAC or the system board is not working properly and will probably need to be replaced.  If the communication failure goes away, then you can try to flash the firmware.  The different ways to flash the firmware are:

Dell Update Package  (includes the SUU)

racadm command line

Through the DRAC web interface

Unified Server Configurator

I think what you mean by "DOS" is using the racadm command line.  That firmware update method needs the Hard Drive file from the downloads site, extracted to a folder:

http://support.dell.com/support/downloads/download.aspx?&fileid=413508

The command you will need is "cadm fwupdate -p -u -d c:\temp", assuming that c:\temp is where you put the extracted files.  If the firmware fails to flash, then the DRAC may be broken.