Any errors associated with this in the logs and or in event viewer?

what is the error message when the backup fail?

No error message shown... just that the tape is full and to insert the next media.  Tested it with the native NT backup and still the same.  Had a look at the tape properties and once the backup has completed it shows the compression ratio of 1.1:1.  Got the same drive installed in three different servers at different sites and they all show the smae compression ratio.  All servers are backing up similair sorts of data but would of thought that the compression ratio would be higher than this.  Checked on the Sony web site and there is no newer firmware for this drive.

Do you have the same results with both HW and SW compression?

If HW and NTBackup also stops at 100GB, I'd think the problem was with ther  drive itself.    But if the diagnostics program can write compressed data, that would blow that idea out of the water

have you called Sony tech support?

 Are you using the Symantec BE drivers or the Sony drivers?

Hi please install the symantec be driver

http://www.symantec.com/business/support/downloads.jsp?pid=15047&type=Driver

and use tools-> wizards--> device configuration wizard
and select use symantec driver

There is a wizard built into the BE console in the menu: Tools->Wizards->Device Configuration Wizard
Select Install Tape Device Drivers and then walk through the pages (being sure to select Symantec drivers).