If your Wii U’s internal memory has failed or become corrupted, you may need to "download" and reinstall the system titles to a new storage medium (like an SD card or SATA SSD).
For a functioning console, "MLC download" usually refers to acquiring games, updates, or DLC to install onto the internal memory. wii u mlc download
Here’s an interesting piece of info regarding : If your Wii U’s internal memory has failed