Backup to LTO-6

Have you installed Symantec Tape drivers for this Tape? This model doesn't appear to be listed in the HCL as supported device (the backups should still work as LTO6 drives are supported).

http://www.symantec.com/docs/TECH205797

Yes i install drivers Symantec for this tape and this model listed in HCL

If a short erase doesn't solve it.  I'd chalk it up to formatting overhead.  

this driver install for my library.

may be because of this?

There is sometimes issues with communication involving OEM drivers.  It is recommended that the Microsoft default drivers are used with a medium changer.  To reset this go to 

1.  right-click the OEM driver > Properties > Update driver.
2. Updating the driver will install the Microsoft driver; Unknown Medium Changer will be displayed

http://www.symantec.com/docs/TECH24414 

What interface (SAS/FC)?  What height drive (full/half)?  Unless you have some weird combination, that library & drive are on the BE 2010 HCL.

IBM LTO6 capacity shows up in my BE as 2.27 TB.  That is based upon what the drive is telling BE, so there may be some rounding involved, or conversion between advertised & engineering units of measurement, or other reasons the drive reports what it does.

It sounds like you are supported and reporting correctly.

I think what is happening is Windows reports it as TiB, and you're reading it from the manufacturer as TB.

2.5TB is about 2.27TiB (right around the 2.3 you see)

So it's all a matter of base 10 or base 2, TB or TiB, etc...  

I use FC and 4 drive half height. Driver installed default Microsoft/

I understand this is a standard condition?