I just purchased a new machine, SuperMicro 1U to upgrade our old UCS installation which was running out of memory. The old UCS installation was installed on a pair of USB drives (using Linux software RAID1).
Apparently the Debian system has a bit of a problem with booting from USB3.0 (XHCI), the xhci_hcd module does get included in initrd but since this modern motherboard is XHCI-only directly into the CPU (no EHCI emulation options in the BIOS either - Intel(R) Xeon(R) CPU E3-1220 v5 @ 3.00GHz), it wouldn't boot saying it couldn't find the drive.
To resolve the issue of Debian 7 not booting from USB3.0 drives I had to boot from the old machine, edit /etc/initramfs-tools/modules and add
above dm-mod or whatever else may be in there, the modules have to be loaded in that order. Then do "update-initramfs -u -v"
The interesting thing is that xhci_hcd does get automatically added to the initrd system when a kernel is installed even if your computer doesn't have USB3 drives and from what I can gather, some BIOS'es support an USB2.0 emulation mode where the drives would be in USB2.0 mode until a driver at the OS level takes over, however it seems manufacturers are now leaving that option out and expecting the OS to be "USB3.0 aware"