A write instruction which affects original data is received via a virtual file system; the original data is read-only on a remote device. The original data is received from the remote device and the write instruction is applied to the original data in order to obtain changed data. The changed data is stored in a buffer in the virtual file system. The changed data is copied from the buffer to a differencing file, where deletion of the changed data from the buffer is independent of completion of copying from the buffer to the differencing file.