Data Compression

If your storage device supports compression, you can tell Backup to use it by turning it on. Normally, you want to leave compression on. The only time you should turn it off is if you plan to use the media in another drive that doesn't support compression. The drive that does not support compression cannot read the compressed data on the media.

Note: You can only change compression when a blank media is in the drive. This prevents the "mixing" of uncompressed and compressed data between sessions on media.

More information:

Compress Data on Media