A disk interface is the circuit board that allows a floppy drive or hard drive to communicate with the computer. If a component on a disk interface board fails and causes it to no longer work, the computer will not be able to communicate with the drive, even though the drive still works. If a drive appears to be in working condition, a data recovery company may replace or repair the disk interface board as a fix at getting the drive to work again.
A user or company who may not have the knowledge, skill, or equipment to repair an interface board can use the interface board from the same model of drive. For example, you could buy the same model of the drive from eBay and use its interface board on the drive you're trying to repair.