A Medium Error is in reference to the media used (i.e. a problem with he disk). However, there are lots of things that can go wrong with the media which is what the 'sense code' refers to. 0x73, 0x03 is to do with laser power calibration.
Every disk brand and type uses different power ranges for the laser and it is down to all sorts of reasons (even the dye used on the disk affects it). Therefore, every disk manufacturer incorporates a section of the disk that the drive reads before any burning takes place. That effectively calibrates the drive's laser for the specific media being used and it does this before writing the lead-in.
Lots of people are suggesting changing the media to rectify the issue but, while this might prove successful because every disk uses different ranges of power, it isn't really solving the problem.
This is a VERY common problem with the Matshita drives Apple choose to use and I have come across it regularly myself which is why I have looked into it. I am no great expert on the subject and am merely stating what I have so far discovered for myself.
There are lots of threads on this topic and people seem to suggest numerous solutions. However, be careful because, as I pointed out earlier, there are lots of things that can go wrong with media and that is what the sense codes are for. So what might work for one sense code does not mean it will work for another, even if they are close.
I do have a few suggestions though. the sense code 0x73 0x03 is basically down to 4 variables, the laser, the power to the laser, the thing controlling the power and the media.
So, use a media you know should work (i.e. one that always has for you in the past). Clean your laser and your drive thoroughly. Check your RAM and if there is anything fishy you should replace it. Reset the PRAM (Shut down then reboot holding Command-Option-P-R altogether until you hear the startup noise for the second time). Finally, and perhaps the most straightforward suggestion, try a different PSU/Battery. Apple power supplies can be dodgy and this has worked for external drives with the same troubles.
However, I am no expert so someone who knows more about drives might have some other ideas.
