Why Do Tape Drives Quote Two Capacities?

Data compression temporarily removes unnecessary data from files in order to make them 'smaller'. Compressing data will effectively increase a drives capacity and enable it to hold more data. Most tape capacities are written as a ratio, with the uncompressed capacity followed by the compressed capacity. For example, a ratio for an LTO-2 drive that could hold 200GB of normal data and 400GB of compressed data would be written like this: 200/400GB.

Why Is The Larger Capacity Not Guaranteed?

The second capacity (400GB in the LTO-2 example above) is not guaranteed and use of this capacity is dependant on the type of data. Different manufacturers will use different ‘average data sets’ (the amount of data that the manufacturer uses to test the capacity of a drive). This may mean that the type of data that you are using may not be compatible with the drive’s capacity and you will not reach the larger capacity.

While simple text-only files can be compressed extremely well, modern image and video files do not compress at all well and may even get larger with compression. Modern programs automatically turn off compression if it would cause an increase in data size or the data is uncompressible (‘Advanced Lossless Data Compression’ or ‘LTO Data Compression’).