I have now made several attempts to burn a Wii game backup on a Dual Layer disc, but to no prevail. I'm using an Apple MacBook, having Windows XP installed on a Boot Camp partition.
My first attepts were done using Verbatim DVD-R DL discs. The first disc burnt perfectly fine, but did not work once used in the Wii. I then learnt that a manual layer break had to be inserted, but after buying a second disc it showed out that DVD-R's cannot handle manual breaks. I continued anyway, which only left me with a "Failed to write sectors 1781088 - 1781119 - Reason: Tracking Servo Failure" error halfway through the burn process.
So I ran out and got myself a fancy DVD+R DL disc, which in turn returned a "Failed to write sectors 2007872 -2007903 - Reason: Unknown (Internal Target Failure) (0x44, 0x90)" error. I have attached the rest of the log entry here: