Need help with Diode circuit

I have a hard drive based MP3 player that requires an even 12v to operate.  I am using a DC to DC converter to take anywhere from 11 to 13.8 and convert to 12vdc.  This application is being used in a boat.  In a boat the unit is used a great deal with the engine off.  When the engine is started the hard drive based unit powers off since the cranking voltage fall below the DC to DC converters lower input threshold.  This is a PITA because the unit must be re qued and set back up again for playlist etc.

It occured to me that I could provide a separate battery to source the HD player that was isolated from the charging and starting circuit.  The only problem is the charging circuit.  If the supply to the DC to DC converter was on the house battery  and an auxillary battery wired in parallel across the input of the DC to DC converter with a Diode in line with the house battery at start up time the auxillary battery would provide the needed current.  With a Diode in line with the house battery the starting circuit should not try to draw power from the auxillary battery and let the DC to DC converter continue to supply power to the HD player.

Did I explain that well enough?  All I want is a small battery to supply the DC converter that will be charged when the engine is running and that is isolated from the current draw during start mode. 

What size and type of Diode do I need and where can I get one?  The HD player draws about 3 amps.  I may add the Head unit to this as well, in that case total amps would be about 8.  Any thoughts apprciated.

Tim T