[CentOS] Centos6: Boot from usb 3.0

Let me try to unconfuse this situation:

1) CentOS-6 does not use initramfstools, mkinitrd is just a thin shell wrapper
on top of dracut

2) dracut by default adds lots of drivers to the initramfs (not only what's
needed, that is, -H is not the default)

3) dracut does not include *hci_hcd modules since ehci, uhci and ohci are
built in, not compiled as modules (this could be considered a bug...).

4) you spelled the driver wrong above, it's xhci_hcd not hdc

To conclude, to build a new initramfs for the current kernel with dracut:

dracut --add-drivers xhci_hcd /boot/initramfs-$(uname -r).img $(uname -r)

To make it permanent look at the add_drivers line in /etc/dracut.conf or add a
/etc/dracut.conf.d/...

/Peter