A disk drive for use with interchangeable flexible magnetic disks of potentially different weights, comprising a disk drive motor for imparting rotation to a disk loaded therein, and a stepper motor for moving a transducer across the track turns on the disk. Since the current consumption of the disk drive motor is subject to change according to the weight of each disk being driven, a current detector circuit is provided for detecting the magnitude of the current being consumed by the disk drive motor. A current control circuit controls the magnitude of the current supplied to the stepper motor in inverse proportion to the detected magnitude of the disk drive motor current. The stepping pulse rate is also varied for making the rotational speed of the stepper motor higher with a decrease in the magnitude of the disk drive motor current. Thus the seek speed is made progressively higher as the disk weight grows less, without increasing the total current requirement of the disk drive.