Consider the following issues when choosing a tape drive:
The single most important consideration. Get a drive that can back up all data on one tape, allowing for some growth. If your data set exceeds the capacity of one tape, you may find that drive is no longer usable (if no one is available to change tapes during an overnight backup) or that it has suddenly become very expensive to use (because you must buy twice as many tapes). In such a case, the only alternative to replacing the drive is to use a backup scheme that mixes full and incremental or differential partial backups, which is riskier for your data.
Most tape drive manufacturers arbitrarily rate their drives at double actual native capacity, assuming you will use software or hardware compression to double the amount of data that fits on a tape. AIT drive specifications go further still, assuming 2.6X compression. The actual compression ratio you experience depends on the data mix (e.g., documents and spreadsheets compress well; executables, images, and archives much less so), the backup software you use, and sometimes on the speed of the computer where the drive resides. We find that real-world data sets typically compress at 1.5:1 to 1.7:1, so plan accordingly.
Having adequate tape drive capacity does not necessarily mean being able to back up your entire hard drive (or drive farm) to one tape in one pass. For example, our network contains more than a terabyte (1000 GB) of disk space ...