I've made some backups, about 1.4GB in size naturally. What do you people use to shrink them (Seen isos at about 300MB), and how good are the results (rank 1-10).
What you're seeing are either compressed archives or wiped image files. RAR files are the most common compressed archives you'll come across, often split into many smaller pieces. A wiped image is a GameCube image that has had all the garbage data removed. While this is fine if you're just storing it on your PC, they're not good for your GameCube. The garbage data is used to pad the game image to prevent excessive wear on the laser diode, so playing with wiped images is a bad idea. All GameCube games should be exactly 1,459,978,240 bytes. Anything more or less is incorrect. Now, if it's homebrew disc, then this isn't as important, as you'd only need about 700MB of data (combination of garbage and useful data) for them to be used safely. If you're trying to save space on your hard drive, get WinRAR and compress images you don't need to burn anytime soon as most GameCube games will compress quite a bit due to garbage data.
