The lost information includes addresses of incidents, phone numbers and patient names, confirmed the service.
It said, "The package containing contact information from our Paisley Emergency Medical Dispatch Centre (EMDC) has gone missing in transit with a courier to one of our specialist IT suppliers.
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"The portable data disc contained a copy of the record of calls to the Ambulance Service in the West of Scotland since February 2006."
The service said all information was password-protected and was also protected by "high security industry standard encryption".
In addition, to further protect the information, "all the data was scrambled so that it should not be possible to match any of the information against an individual", the service said.
The extensive protection of the data will be commended in some quarters, but the HMRC lost disks debacle saw the public sector being slammed for sending disks by courier, instead of securely sending data electronically.
The disk was on its way to IT systems firm MIS Emergency Services in Manchester, but was reportedly lost by courier firm TNT.
A helpline to re-assure past callers has been set up.