The main reasons that you have lost the data ...

Raid arrays.

1. Incorrect administrator actions

Work raid array 5 levels without a single disk.

Using raid array level 0 (stripe) without creating copies or mirroring (raid array level 1).
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Edit the raid without previously created sector-by-sector copies of the hard raid.

Refusal to use backup devices

Lack of antivirus protection or any other protection from external exposure.

Lack of monitoring equipment performance and maintenance work.

Random errors.

2. Equipment failures.

Use of uncertified or untested equipment.

Use of equipment in extreme conditions (climatic, violation of technical conditions of operation - instability of the network, dust, vibrations, etc.)

Exceeding the life of the equipment.

Hard drives.

1. Incorrect user actions.

Inappropriate use of specialized programs.

Repartitioning arrays with incorrectly running obsolete programs (such as Partition Magic)

Refusal to use backup devices

Lack of antivirus protection or any other protection from external exposure.

Installation of new programs and their use.

Random errors (random formatting, system reinstallation).

2. Equipment failures.

Use of uncertified or untested equipment.

Use of equipment in extreme conditions (climatic, violation of technical conditions of operation - instability of the network, dust, vibrations, etc.)

Exceeding the life of the equipment.

Flash drives.

1. Incorrect user actions.

Refusal to use backup devices

Lack of antivirus protection or any other protection from external exposure.

Installation of new programs and their use.

Random errors (random formatting system reinstallation).

2. Equipment failures.

Use of uncertified or untested equipment.

Use of equipment in extreme conditions (climatic, violation of technical conditions of operation - instability of the network, dust, vibrations, etc.)

Exceeding the life of the equipment.

Overview based on Tetronix VT statistics http://www.tetroniks.ru/ (statistics for 8 years, average 900-1000 orders per year)