A police memory stick containing sensitive data about serious crimes and witnesses was stolen during a burglary at a detective's home, it has emerged.
The unencrypted device with no password protection stored details of 1,075 people with links to drug probes, arrest targets and officers' names.
Greater Manchester Police has been fined £120,000 by the Information Commissioner's Office (ICO)
The device was stolen in a burglary at a detective's home in July 2011.
In a report, ICO director of data protection David Smith said if the data got into the hands of "untrustworthy third parties" it could expose the people involved to physical harm.
He said: "This was truly sensitive personal data, left in the hands of a burglar by poor data security.
"The consequences of this type of breach really do send a shiver down the spine.
"It should have been obvious to the force that the type of information stored on its computers meant proper data security was needed.
"Instead, it has taken a serious data breach to prompt it into action."
Mr Smith added: "This is a substantial monetary penalty, reflecting the significant failings the force demonstrated.
"We hope it will discourage others from making the same data protection mistakes."
A similar security breach took place in September 2010 but the force failed to sufficiently re-train staff and put precautionary measures in place, the ICO said.
Ass Ch Const Lynne Potts said: "This was very much an isolated incident.
"We take all matters relating to the storage of data extremely seriously and have stringent measures in place to ensure the safe storage of data."
It is understood the detective was internally disciplined and that all those whose names appeared on the device have been contacted.