During OSD development one of the most useful things you can do is to enable command prompt support to allow you to open up a command prompt and perform troubleshooting. This is simply a matter of turning it on in the Boot Image as per https://blog.thesysadmins.co.uk/sccm-2012-sp1-enable-command-support-console-winpe.html
If you are used to MDT you may be puzzled to find that the equivalent in the Windows stage (Shift + F10) is missing when you deploy via SCCM. This is because SCCM sets a flag to prevent this from being accessable.
To regain this prompt, after the applying OS stage and you have rebooted into Windows delete the flag file which is located at: