The drive containing patient names, birth dates and medication information was last seen in a secure area of the nuclear medicine department at the Anaheim location in September. The drive may have simply been misplaced, officials said.
The health care provider says no social security numbers were stored on the drive, and criminal activity is not believed to be involved.
Patients were notified of the incident last week and are being encouraged to review their medical statements.
The Associated Press contributed to this report.