Over 4,000 officers had their personally identifiable information leaked.
ZD Net explains:
A unsecured backup drive has exposed thousands of US Air Force documents, including highly sensitive personnel files on senior and high-ranking officers.
Security researchers found that the gigabytes of files were accessible to anyone because the internet-connected backup drive was not password protected.
The files, reviewed by ZDNet, contained a range of personal information, such as names and addresses, ranks, and Social Security numbers of more than 4,000 officers. Another file lists the security clearance levels of hundreds of other officers, some of whom possess "top secret" clearance, and access to sensitive compartmented information and codeword-level clearance.