This is not a Dual Layer capable DVD burner.
This is a hardware limitation that will not be cured by a firmware update.
Sorry
P
Here is my Mac mini information:
System Profiler > Hardware > Disk Burning:
MATSHITA DVD-R UJ-845C:
Firmware Revision: DPP9
Interconnect: ATAPI
Burn Support: Yes (Apple Shipped/Supported)
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -RW, +R, +RW
Burn Underrun Protection CD: Yes
Burn Underrun Protection DVD: Yes
Write Strategies: CD-TAO, CD-SAO, DVD-DAO
Media: No
If it doesn't support DL, would a firmware update fix this or is it a hardware limitation? Thanks!

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