Backup job started to fail due to backup size

Dear All,
I am using backup exec 2010 for the Backup Jobs with HP Ultrium-3 drive drive.
I am using Media C7973A 800 GB RW Ultrium-3  LTO

all of sudden the backup job started to fail due to backup size, I checked on the media properties, statistics and found that the total capacity is 391 GB !!!

Media type is set to LTO , I  changed it to LTO 4 (800 GB) but with no effect.

I used HP Storage works library and tape tools and the capacity is seen as 800 GB.

Kindly advice why the media is seen as 391 GB only in Backup Exec ??