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