When you back up a compressed volume, files are written to the backup media in compressed or uncompressed format, according to how they are currently stored. To back up compressed files in uncompressed format, you must decompress the files first, then back up the files.
When you recover a volume, the files are restored in their saved format to the destination volume. If you try to restore a compressed file to a volume without compression, the file is not readable. For information, see NW 6.5 SP8: Storage Management Services Administration Guide, or see your third-party backup application’s documentation.