Hi,
I had that problem on my older MB and now, after upgrade to Maximus VII Hero I have it again.
I have 6 disks in my PC (4x HDD, 2x SSD) + DVD rom, so I have to use 7 sata ports, and the one disk thats in AsMedia port is just dissapearing from the system during some load (IE watching BluRay movie from it). To get it back I have to restart my PC.
I installed all drivers, and I really have no idea what should I do next.
Ok, so now Im not sure whats happpening, as the disk that dissapeared should be plugged in Intel sata port....
It wasnt even shown in BIOS, but when I booted to windows, its there.
Basically it does same stuff as it did on my last MB with Marwell ports.