It's missing from the State Street Campus in Bangor and includes information about patients who received cardiac treatments between 2011 and 2017.
The hard drive is operated by a third party, and includes the patients name, their conditions and other information.
It did not include their addresses or social security numbers.
The hospital says although the risk of identity theft is low, they'll be sending out letters to affected patients.
They'll also be offered free identity theft protection for one year.
An assistance line has also been established for people seeking information at 1-855-973-8200.