An apparatus and method for forming plugs in a wellbore, according to which a first volume of slurry is introduced into a work string in a wellbore so that it flows through the work string and discharges before hardening to form a plug. The tracking circuit receives an input voltage and a reference voltage and selectively biases the first transistor to one of the input voltage and the reference voltage based on a comparison of the input voltage and the reference voltage. Lighting units may be positioned between vertically adjacent rails. The second side panel defines a groove for slidingly receiving a screw fixed to the data storage device.