This error can occur when attempting to recover an encrypted USB disk using the pgpwde --recover command from the command line. This can occur on USB disks which contain an extremely large amount of data prior to encrypting the disk.
You can recover the data on the disk by using a Windows PE disk or by using a PGP Whole Disk Encryption recovery disk for your version of PGP Desktop.
For information on the PGP Whole Disk Encryption recovery disk, see the following two articles: