A cpio(1) archive has either become corrupted or was written out with an incompatible version of cpio(1).
Use the -k option to cpio(1) to skip I/O errors and corrupted file headers. This might permit you to extract other files from the cpio(1) archive. To extract files with corrupted headers, try editing the archive with a binary editor such as emacs(1). Each cpio(1) file header contains a filename as a string.
For more information on magic numbers, see magic(4).