Read the user guide:
The only times you can resize cartridges are when you first create a library or when you add slots and they are blank. Once the cartridges are made available to the backup application, it formats them ready for use and they are no longer blank, even if you have not yet written data to them.
Similarly, the erase function on most backup applications does not return the cartridge to a completely blank state.
The Max Size defaults to the appropriate capacity for the drive emulation type selected on the Devices tab, but if the Used Size is Blank, you can edit this value. The maximum value allowed is 1600 GB.
Click Edit for the appropriate slot. Select a value from the drop-down menu and click Update.
NOTE:
The maximum size indicates the physical disk space available for that slot assuming uncompressed data; it will only be committed when data is written to it. Changing the maximum space does not affect actual disk usage or save disk space, but may be useful if you know you will be moving data to a physical tape with a smaller cartridge capacity.
Just read the labels on the media correctly. An Ultrium 4 is 800 GB, not 1600 GB.
There's an * leading to a note saying that 1.6 TB is assuming data can be compressed 2:1. This has been like this since DAT drives 15 years ago.
