How Tape Backup Works

Tape backup works by reading specified files on your hard disk and copying them to a series of tapes. The contents of the tapes are compared to the contents on the hard disk to determine if copying errors have occurred. Good tapes are removed to a safe location where, hopefully, they never need to be used. After several sets of tapes have been made, old sets are returned for reuse.

Files are accessed differently on tapes and hard drives. Hard drives allow random access to positions on the disk; tapes offer sequential access. Hard disks have directories which allow a file to be reached in milliseconds. Tapes cost less than a tenth of a similar size hard disk. These differences mean that you will use tapes differently than disks and you will learn a different set of capabilities and inconveniences for the two media. The most important thing to learn is that although you can append to a tape without disturbing it, you will lose all the data on a tape when you initialize it.

QIC tape drives don't require formatting or erasing before they are used, or reused. Although you might want to retension a tape that has been sitting a long time, you can rewind a tape and begin writing immediately. The reason a format operation is not necessary is that the format and erasure occur as the tape is being written. Whenever writing occurs on the first track of a tape, a full width erase bar is energized. It erases all of the data on all of the tracks just ahead of the write head. When subsequent tracks are written the erase bar is turned off.

This design produces one important side effect on QIC tapes. Once you write the initial 512 byte block, all the data on the remainder of the tape is lost. Even though the tape drive's write head has not yet recorded over the data, the data is lost.

You really don't have a second copy of your data until your tapes have passed the verify operation, and when you initialize a tape, you don't have a second copy at all. All this trivia means a minimally safe backup procedure requires at least two tapes and an adequate one requires three.

When properly installed and used on a regular basis, SecondCopy will produce a safe, second copy of all the files on your hard drive. In the event that data is lost, SecondCopy will allow you to restore an earlier version of the lost data. SecondCopy is also used to make duplicate copies of hard disks for software installation, and to defragment hard disks for improved system performance.