Many people have a basic understanding of the principles of hard-disks and data recovery from disks. Tapes, however, seem to be more difficult to grasp, while both tape and disk store data magnetically. This would make tape and disk principally similar.

The fundamental differences between tape and disk derive from the fact that disks store data on rigid, circular platters, while a tape stores data on long narrow coiled up flexible material. The disk spins all of the time and can give you access to any sector on the surface in a fraction of a second. We call this a random access medium. The tape, on the contrary, is a sequential storage medium. The tape moves only when it needs to access data and if the data is stored on the other end of the tape it could take several minutes. Furthermore, we do not usually take the platters out of a disk. A tape drive is designed so that you can continue a new tape when one is filled up, so the recordable media must be removable. Another major difference for the user is the way files are accessed. You can use Windows Explorer to access anything on a disk, while this is not normally possible with tape. Put an LTO or DLT in your drive and if it was not written using a piece of backup software that you have installed then your chances of even identifying what is on it are remote.

These different media have their own pros and cons. It does not mean that one is better than the other; they just serve a different purpose. Disks are suitable for fast access to any section on the surface and meet the demands of real time computer activities. Tapes are more durable and cost-effective in long term archival storage and data recovery. Imagine using a tape for your daily computer activities. It would take ages to load a program and open a file. And vice versa, you could use a disk for archiving

data, but the cost per megabyte storage is not favourable with disk, and is it wise to risk your archival data by storing it on a medium that if dropped might never work again?. With hard disk, if the heads hit the platter then it may well be game over.

Other differences between tape and disk are not as easy to comprehend, but nevertheless very important to the data recovery specialist.

Tapes encode non-data entities that you won’t find on disks. These entities include File Marks and Set Marks, which are used as dividers for easier access to a selected range of data blocks. On a tape, a block size can be fixed or variable and in theory even go as high as 16MB. On disks, there are sections of usually 512 bytes per section. It is possible to format them with different sized sectors, but all sectors will be identically sized.

End of Recorded Medium is not a concept found with hard disks. A hard disk has an amount of space available and all is accessible for use. Tape, has an amount of recordable media available but reading is only permitted up to the furthest point where data has been written. You write 20MB to a 300GB disk, you can still read 300GB of data from the disk. You write 20MB to a Tape, all you can read is 20MB from that tape. One major area of tape data recovery is the recovery of data following overwriting and tape re-initialization.

Overall, even though tape and disk may seem similar, the differences are major. This is why they are used for different purposes and each serves the purpose very well.

Michiel Van Kets provides articles for Mark Sear, who has been working in the field of Data Recovery since 1984 and later formed Altirium, a company which offers disk recovery and data conversion services for tape, disk and all other computer storage media.