Most likely what you are running into is XP Embedded's Enhanced Write Filter (EWF)
Enhanced Write Filter (EWF) provides the ability to write-protect a run-time image. By redirecting all write requests to either a separate disk partition or RAM, EWF allows the run-time image to maintain the appearance of a writeable run-time image. Additionally, Enhanced Write Filter provides the ability to deploy a run-time image onto read-only media, such as a CD-ROM.
You can turn it off using the EWF Manager Application (if it was included in the Embedded image).
To start the EWF Manager run
Ewfmgr, and use it to get status, enable and/or disable the EWF, etc.
More info from MS here: Using the Enhanced Write Filter (EWF) in Windows XP Embedded