Modding consoles in order to play their libraries off of SD cards/external drives, or to do the same with backwards-compatibility support features, can be a headache. Not in terms of effort to perform the mod, but in terms of understanding where to even begin. I've realized this every time I decide to mod a system myself. Forums and ebay listings throw around unique terminology so casually, leaving you to figure it out what parts and processes are needed for yourself. So I'm going to give a very short breakdown on the proprietary terms and concepts you'll need to know so you'll have an idea of what and why stuff is needed before you go down the rabbithole.
Play your whole Gamecube library off an SD card:
For this, you'll need to boot your Gamecube into an environment (from a special disc) that can read from an SD card and let you choose which game you want to play. The Gamecube won't boot burned or region-locked mini-DVD's stock, so in order to run this special operating environment, you'll need a modchip to tell it how to as it starts up.
Xeno: The modchip that tells your Gamecube how to (and that it's allowed to) boot from non-game discs
SD2SP: A device that uses one of the accessory ports on the bottom of the Gamecube to take an SD card and give your special boot disc environment storage to either boot games from or use as a game save device (memory card).
Memory Unit SD Card: A modified memory card (that plugs into your normal memory card slot) that also holds an SD card for the boot disc environment to read as storage
Swiss Boot Disc: The boot disc that contains the special environment to read the SD cards and boot+save games off of them.
.DOL Files: Dolphin boot configuration files- they're what you use to boot into the boot environment with specific configuration parameters.
Custom memory unit: A memory card that has .dol files on it already to boot into a specific environment, that the Gamecube can boot into when a specific game disc (custom memory units come in a bunch of flavors that work with a single game when it boots- like SSB, Zelda Wind Waker, etc. in an effort to make it convenient for the buyer and use whatever game they might already own).
Alternative method terms:
GCloader: a pretty expensive solution that replaces the optical drive in the Gamecube with an SD card reader directly.