Ah, my mistake, I'm getting mixed up between minidiscs and the 8cm mini CDs.

You can get multi-layer M-disc BD-Rs, though, up to the triple layer 100GB BDXL (although you need one of the BDXL burners to write those; the 50GB BD-R DLs can be written by most burners). They cost a pretty penny, though!

The biggest problem now is the disappearance of Bluray burners/writers. Here in Aus there are no internal drives available on the market any more. I've had to stock up with a few second-hand spares before they get too pricey.