One of my own KB articles is published on the knowledgebase. If you have this error then check out if this can solve your problem. If not, check out the end of the post for more troubleshooting tips.
Error: vDisk is not available. Please check your network PXE boot Configuration and restart imaging wizard.
Running the imaging wizard on a Virtual Machine, which is hosted by XenServer and the XenTools are updated or reinstalled.
When trying to update or create a new vDisk and running the Imaging Wizard on a Virtual Machine, you receive this error when the Virtual Machine is restarted and the imaging process should start.
This can happen if the XenTools are upgraded to a new version or are removed and reinstalled after the Provisioning Services Target Device Software is already installed.
When you get this error, you must remove and reinstall the latest Target Device software, which is provided with the version of Provisioning Services you are running.
After the Target Device Software is reinstalled and the Virtual Machine is restarted, you can start the imaging wizard again and after setting all options and restarting the Virtual Machine, the imaging process should start as required. If you still receive the error, refer to the Knowledge Center articles in the More Information section.
CTX133272 – Error: vDisk is not available. Please check your network PXE boot Configuration and restart imaging wizard.
CTX131582 – Error: "vDisk is not available. Please check your network PXE boot configuration and restart Imaging Wizard." When Using Image Builder or XenConvert
CTX124185 – Provisioning Services Antivirus Best Practices
CTX117374 – Best Practices for Configuring Provisioning Server on a Network
You can read the original KB article here.