Double Layer DVD problem - help

[William W. Austin]

Simple question (I posed it previously but probably wasn't clear)

I have a dual-layer dvd burner (Sony DVD RW DRU-720A) which I got to  
replace my old dvd burner.

I can successfully burn single layer dvd's with no trouble on this  
drive - and after testing it works fine under windows.

However every dual-layer dvd I've burned so far is now an expensive  
coaster.  I have reread the docs I can find, but don't see any  
additional options for the dual-layer dvd's ...

If somebody could give me some help on this one I'd appreciate it.

Notes:  same results (failure) with both mkisofs ... cdrecord and just  
using k3b.  Tried on both DVD-R DL and DVD+R DL.

cdrecord-2.01.1; kernel-2.6.12-1.1372_FC3; minimal system activity;
no underruns reported at all.

My normal script is simple (here's the stripped-down version):

     #!/bin/sh
     cd ${1}
     mkisofs -o /<somewhere>/XXX.iso -R -f -T -v -A \
	"XXX" -V "XXX" .
     (mount and verify the iso image, then unmount it)
     cdrecord --dev=/dev/hdc -v -dao speed=16 -data /<somewhere>XXX.iso

resulting single-layer dvd-r works fine.  double not recognized and  
won't mount on any drive (on several machines with differing OS's).

I presume I'm missing an option here some place, but haven't found it  
yet...  so someone please give me a clue if you can.

Thanks,
   william austin
     waustin at speakeasy.net