Wikipedia
Hardcard is the genericized trademark for a hard disk drive, disk controller, and host adapter on an expansion card for a personal computer.
Typically a hard disk drive (HDD) installs into a drive bay; cables connect the drive to a host adapter and power source. If the personal computer lacks an available bus on a compatible host adapter, then one may have to install an adapter into an expansion slot. The Hardcard supplies its own host adapter, and doesn't require an empty drive bay.
Plus Development, a subsidiary of Quantum Corporation, developed the first ISA Hardcard, and released it in October 1985. By June 1986, 28 of Plus Development's competitors were producing similar products. The term has been used generically to refer to any hard disk on a card.