This morning I tried to install CentOS7 on an Apple MacMini from 2011 with a USB stick. After grub and the initram fs has been loading there was the following error
dracut-initqueue: Warning: Could not boot. dracut-initqueue: Warning: /dev/root does not exist. Starting Dracut Emergency shell.
This error often occurs by a mismatched usb media label and can easily be resolved by using UUID instead of label in grub2 initram fs kernel arguments .
The following steps have been taken form unix stackexchange on how to fix this:
1. starting from the Dracut Emergency shell, find out which device name is your installation usb stick.
2. get the uuid for this device name, write it down.
3. reboot and replace grub2 initram fs kernel arguments from using LABEL to UUID.
The exact procedure can be found here
4. start the installation with the changed kernel arguments, this is a bit tricky because running a changed grub2 command line needs the keyboard combination ctrl-x which does not work on a mac computer .
This also does not work when using a PC keyboard on this mac.
Solution was to use a PC keyboard and type F10 instead of CTRL-X work the same on mac to run a changed command line. I tried pressing F10 on a mac keyboard (fn+F10 key) but did not work.
But using a PC keyboard on the mac and hitting F10 worked and booted up the graphical installer.