According to most websites, Ritek mini DVD-R's are the best format for the Modded Gamecube for using as backups.
Well, I have tried the following without success (without tweaking the laser)
Benq - Full size DVD-R
Maxell - Full size DVD-R
Memorex - Full size DVD-R
Fujifilm - Full size DVD-R
Nexxtech - full size DVD-R (from the Source - AKA Radio Shack)
Verbatim - mini DVD-R
But, I tried a brand known as Ridata (full size discs). SUCCESS!!!!
Turns out that Ritek as goes under the name of Ridata and as is the brand name behind "The Office Depot".
These disks seem to work everytime without a problem.
I also was at a dollar store and thought I would try one more brand name just for the hell of it. So for a buck I tried a disk that went under the name of Imation (www.imation.com). Once again, SUCCESS!!!
Anyways, for those of you having no luck with media backups give Ritek (AKA Ridata, AKA The Office Depot name brand) or imation a try.
Hope this helps with burning through some coasters in order to get your backups made.