Design Considerations for Using EWF with Device Update Agent (Windows Embedded Standard 2009)
If you are protecting a device with Enhanced Write Filter (EWF), but want to be able to apply updates and patches to your run-time image, you can use the Device Update Agent (DUA) feature in conjunction with EWF.
Because EWF prohibits write access to the device, when you want to apply an update by using DUA, you must use one of the following procedures:
Run DUA as a background service on the device and let it poll for updates. When you author your DUA script, add a command that uses EWF Manager to either disable EWF or commit the EWF overlay. Committing and/or disabling the EWF overlay will allow you to write to your run-time image.
When you author the Device Update Script, it should start the EWF script that allows you to write to your run-time image.
For more information about the EWF Manager command, see EWF Manager Commands. For more information about updating an EWF-protected run-time image, see Installing Updates on an EWF-Protected Run-Time Image.
Optionally, you can leave the DUA service disabled on the run-time image. When there is an update available, you can manually disable EWF, start the DUA service, and allow DUA to apply the updates. When the DUA update is complete, you can restart EWF.
Additionally, you can use DUA to run a program that uses the EWF APIs to enable and disable EWF programmatically. For more information, see Enhanced Write Filter API.