The recovery of data from backup tapes is a broad area, too much so to be entirely covered within this page. The types of problems are broadly the same across the different types of backup tape, though the specifics differ widely.

Broadly the types of problems requiring tape data recovery are as follows.

Medium Error/Media Flaw

This is the error that is reported when a request to read data from the tape fails because the drive cannot access data at that position along the tape. The actual problem can result from media degradation (this can just happen, or result from poor storage environment), physical damage, or a recording made by a failing drive. If a tape drive’s write heads are failing or excessively dirty, or the alignment of the tape relative to the recording heads is incorrect then the result can be an area of tape that cannot be read.

Most backup applications cannot cope with read errors and will terminate the restore operation at the first error. Even if the application were to continue in its attempts to restore data many drives will not clear the assertion of medium error until the drive has been rewound, and in some cases the drive must be unloaded and reloaded.

Overwriting and re-initialisation (or erasure)

Unlike a video or audio tape where if a recording is partially overwritten by a shorter recording and the part of the recording that has not been overwritten is still available, computer backup devices prevent access to anything but the latest recording. Each drive handles the matter in a different way, but the principle is the same across them all. If you have a tape with 500GB of data and write 20KB at the start of the tape, then there is a presumption that the original 500GB is no longer wanted. Following the new recording an end-of-data (EOD) is written to the tape and that point cannot be passed without access to tape data recovery facilities.

It would not help much if you could get passed as the backup application would not be able to understand what it found. Backup applications expect a sequence of data that starts with a known set header and then follows the correct sequence of information about data and data, landing in the middle of a file is not something that most applications can handle.

Damage

With sever damage, such as the start of the tape being crumpled in the drive mechanism or the tape being snapped then attempts to load a tape and access data with do anything from nothing to causing further damage and might well result in a tape being stuck in the drive and tape mangled up within it. Attempts to fix tape without understanding the underlying recording format will often result in further damage.

Less severe damage, such as creasing part of the way along a tape, will usually result in the same symptoms as with a medium error. With many types of tape, however, recording runs sequentially from start to end, then end to star and up and down the tape many times. Physical damage will usually cross several data tracks and so will have the effect of causing data loss and restore failures at several positions throughout a backup sometimes making the tape recovery process rather tortuous.

There are sometimes things that you can do for yourself, but with caution. If there is an error reported during the restoration of a file then it might be that opting for a selective restore that does not include that file might get the data that you need. Attempting the restore operation using a different drive might work where there is a marginal recording problem or minor alignment issue.

The key thing is to remember that unless you know precisely what the cause of the problem is, and with a tape that is tightly wound on to a spool you cannot see if there is damage, then attempts to read from the tape could make matters a lot worse and result in the loss of data that could otherwise have been recovered.

If you have a problem with a tape then click here and get a technical answer to any questions you have.