First, this should be in the NEC/Optiarc forum, someone will hopefully move soon.
Power Calibration Error is a usual sign of poor quality media. RICOHJPN D01 is very poor quality, and unless sold by certain brands, is fake Ricoh media, usually made in China/Hong Kong.
The 7240 is a good dual layer burner, and I (as would most others) would suggest using Verbatim DL Media, as is is the only readily available and most importantly the most reliable DL media.
You cannot set the 7240 to write these discs at 2.4x, as 4x is the minimum supported speed. The only way round this would be to use MCSE and swap write strategies in the firmware.