When the drive is damaged physically.
One of the most common reasons for data loss is physical damage. A simple manufacturing defect, or external influences such as shock, dust or power surges may cause internal damage. Anti-static discharge or ESD is also another common cause of failure. Physical damage will inevitably require the help of a data recovery expert. Unless you have a clean room and donor parts, any attempts to recover the data yourself will almost certainly render the data unrecoverable. A data recovery expert will assess the damage to your media before they can determine what parts of it may need to be replaced in order to have the drive working again. Often the expert will use a duplicate hard disk drive of the same make and model to replace damaged parts. Reputable experts will provide a file listing of files and quote for the recovery beforehand.
When the operating system fails.
Luckily if your operating system fails, it means that nothing is wrong with the hard drive itself. The data areas are usually safe, therefore your chances of a full recovery are high. All the expert needs to do is to copy the data from your hard drive to a new one. This can be easier said than done, so take advice. If you are not familiar with hard drives and data structures, don't risk your valuable information.
When it dies of old age.
As hard drives grow old they experience unpredictable failures. All hard drives will fail at some point, but you will never know when. The mechanical parts of a hard drive will wear down over time and the media surface degrade. Eventually this will cause the drive to crash. Hard drives are becoming increasingly more reliable, but always read the manufacturers ‘mean time before failure' or MTBF. This is normally calculated in hours and when this has expired, it might be time to upgrade.
If it catches a virus.
Computer viruses can be extremely malicious and damaging to your computer, whilst ransom-ware can be expensive if you fall victim. It is not only recommended but necessary to have a decent anti-virus program installed on your computer. Many users forget that malware doesn't always present itself as a virus. So anti-malware software such as Malware bytes should also be considered essential. Some viruses will quickly spread around your computer and onto any networks your computer is connected to. So even if you have a mapped backup, catch any malware quickly before it spreads. Make sure you are extremely careful when downloading files and applications online, ensuring they are from safe sources, and don't open any files emailed to you from an unknown source.
When you accidentally delete or overwrite data.
Manually overwriting data or accidentally deleting it is a common story. Many people will often think their data is already saved somewhere else and will delete the files. While data being overwritten is usually an accident, it is often intentionally done. In criminal and forensic circumstances data may be intentionally erased in an attempt to cover up the tracks of illegal activities, meaning forensic experts may be hired to perform data recovery for the hidden data. Unfortunately, it can be difficult and sometimes impossible to recover data that has been overwritten.
The best way to avoid data loss, is by simply backing up your valuable files on external hard drives or making use of cloud services. If you choose to back up your data, make sure to store it in a safe and secure location away from your computer. This will ensure any damage such as flood, fire or theft will not affect your backed up files. Also make sure that you are saving whole copies of the original files and remember to back up regularly.