I am using HP LTO-4 Ultrium tapes which has a native capacity of 800 GB/1.6 TB comnpressed. Why is it showing an available space of only 102400 MB as shown below? I double checked the tape and nothing has been written in it. When I try to format a tape, in the Medium size field, it only shows a capacity of 102400 MB.
you can specify any size you want, but that's just cosmetics. The tape will be written until full, regardless of what you write here. After being written to full capacity once, the written capacity will become the new estimate anyway. So just use the default, it's not worth your time to micromanage that value. There's no correct value to write there anyway, compression ratios vary wildly.