My Backup tape is 400GB, but I can only get 300GB on a tape. Why?
Because the people who make tape drives are technical people, and the people who sell tape drives are marketing people.
Tape backup drive manufacturers refer to the capacity of their drives by quoting the amount of compressed data that can be held, based on an assumed compression ratio of 2:1.
In reality an LTO2 tape actually has a 200GB capacity, but is referred to as 400GB, or sometimes 200/400GB. An LTO3 tape actually has a 400GB capacity, but is referred to as 800GB, or sometimes 400/800GB. An LTO4 tape actually has a 800GB capacity, but is referred to as 1600GB, or sometimes 800/1600GB
The assumption is that when you write data to the tape, you will compress it, and the data will compress at a ratio of 2:1. The actual compression rate you will achieve depends upon several factors, but primarily the type of data being backed up. For example, if you are backing up plain text files, you should find that they compress well, whereas MP3 files do not compress (they are already compressed).
In Backup Exec's media view you can see the compression rate for the media you have written to.