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Überschreiben (Overwriting)
Anyone who deletes a file or formats a hard drive usually only removes the references to the data. The data itself remains until it is overwritten. Targeted overwriting is therefore the classic method for sanitizing hard drives before they are passed on. Here you will learn how it works, which variants exist and where its limits lie.
In brief
Overwriting is a software-based erasure method in which all storage areas of a data carrier are written with new data such as zeros or random values. The original data is thereby replaced and can no longer be read by normal means. For magnetic hard drives the method is proven; for SSDs it is often not enough on its own because not all storage areas are reachable.
How overwriting works
Erasure software addresses the data carrier directly and writes a defined pattern sector by sector. This can be a sequence of zeros, a fixed bit pattern or random data. Afterwards, good software checks whether the pattern is actually present everywhere. This check is called Erasure verification. Only with it does an attempt at erasure become a verifiable result.
It is important that areas the operating system does not display are also covered. These include hidden areas such as the Host Protected Area (HPA) and the Device Configuration Overlay (DCO). Defective sectors that the hard drive has replaced internally are called reallocated sectors. They can no longer be written to via the normal interface.
Well-known overwriting methods
| Method | Passes | Classification |
|---|---|---|
| Simple overwriting | 1 pass with fixed pattern or random data | Generally sufficient for today's magnetic hard drives |
| DoD 5220.22-M | usually 3 passes, often with verification | Older US method, still frequently named in tenders |
| VSITR | 7 passes | Former German government agency standard |
| Gutmann method | 35 passes | Developed for old hard drive technology, no added value today |
| Firmware commands (Sanitize Overwrite, Secure Erase) | Executed by the drive itself | Also reaches areas that software cannot see from the outside |
More passes do not mean more security on today's hard drives. The US guideline NIST SP 800-88 states in its 2014 version that for magnetic data carriers, a single pass with a fixed pattern generally prevents recovery. The 2025 version refers to the IEEE 2883 standard for technical methods.
Limits and risks
- SSDs and flash memory: The controller of an SSD distributes write operations across changing memory cells and keeps reserve blocks. Overwriting from the outside therefore does not reliably reach every cell. Here, firmware commands or crypto erase are the better choice.
- Defective drives: A data carrier that cannot be written to completely cannot be overwritten securely. It must be physically destroyed.
- Time required: Overwriting large hard drives takes hours.
- Evidence: Without a report per serial number, the erasure cannot be proven in an audit.
Rule of thumb: With hard drives, what counts is not the number of passes but that all areas are really covered and verified.
Overwriting in IT retirement
In practice, professional erasure programs combine overwriting with firmware commands and verification. Which method suits which drive is explained in our guide Securely erasing hard drives and SSDs.
At Second IT, HDDs, SSDs and mobile devices are erased with Blancco software, including hidden areas such as HPA and DCO. The erasure meets international standards such as DoD 5220.22-M and NIST. For each device there is a digitally signed erasure certificate with serial number. Data carriers that cannot be erased in an audit-proof way for technical reasons are destroyed in-house in accordance with DIN 66399. More on the certified data deletion.
Frequently asked questions about overwriting
How often must a hard drive be overwritten?
With today's magnetic hard drives, one complete pass is generally sufficient if all areas are covered and the result is verified. Multiple overwriting dates from the era of older hard drive technology.
Can an SSD be erased securely by overwriting?
Not reliably. Because of internal reserve blocks and the distribution of write operations, overwriting does not reach every memory cell. Firmware commands such as Sanitize or a crypto erase are suitable for SSDs, each with verification.
Is overwriting the same as formatting?
No. A quick format only creates a new, empty file system; the old data remains. With overwriting, the data itself is replaced by new values.
Why do hidden areas also have to be covered when overwriting?
Areas such as the Host Protected Area (HPA) and the Device Configuration Overlay (DCO) are not displayed by the operating system. If they remain untouched, data can be left behind there. Firmware commands also reach areas that software cannot see from the outside.
What happens to hard drives that cannot be completely overwritten?
A data carrier that cannot be written to completely cannot be overwritten securely. It must be physically destroyed. This applies, for example, to defective drives. You should also receive proof per serial number for this.
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