There's a nice description of the product here:
"The Sundriver is the first modchip with full HDD support with a fully concealed install. It allows you to play backups off an internal SATA 2.5" hard drive. Since this is a hardware mod you do not need to worry about patches and bricking the Wii like you do with softmods. There is zero soldering involved and everything is plug and play. This chip is fully upgradeable. The entire chip and hard drive is installed inside the Wii. This is done by replacing the DVD drive compartment with the Sundriver + HDD."
The feature list reports that no homebrew software is required for this to work. I assume therefore that the device emulates a GC game which, when launched, presents the device's menu for ISO mounting. Any it's less than 50EUR. This is very tempting!