Off-line backup storage takes a step beyond hard drive storage in terms of capacity (higher) and speed (slower). Here, capacities are effectively limited only by your ability to procure and store the removable media.
The actual technologies used in these devices varies widely. Here are the more popular types:
Of course, having removable media means that access times become even longer, particularly when the desired data is on media not currently loaded in the storage device. This situation is alleviated somewhat by the use of robotic devices capable of automatically loading and unloading media, but the media storage capacities of such devices are still finite. Even in the best of cases, access times are measured in seconds, which is much longer than the relatively slow multi-millisecond access times typical for a high-performance hard drive.
Now that we have briefly studied the various storage technologies in use today, let us explore basic virtual memory concepts.