I have grub2 installed on my PC's internal hard drive and I am trying to boot a debian installation on a USB key. The PC doesn't have any USB 3 ports so I have added a USB 3 PCIe card. I know that most PCIe USB3 cards don't have boot ROMs, so my plan was to load grub2 from the hard drive and then boot into the USB key.
When the USB key is plugged into a USB 2 port everything works fine in the setup. When I try to instead plug into the USB3 port on the PCIe card, the following error appears:
Blockquote error: no such device: UUID of drive here
After that grub goes into the rescue mode and ls only shows (hd0), (hd0,msdos1), and (hd0,msdos2). So it's as if grub can't "see" the usb key.
I've added insmod for usb, usbms, xhci, echi, uhci to /boot/grub/grub.cfg . I know the USB 3 port works from linux because when I run from a distro on the hard drive I am able to mount from usb keys in the port. Is there any way to get grub2 to support the PCIe USB ports?