loaded with a diskette d 5 1 during the next period t6 d 1 1 is removed from drive 1 and a new diskette d 1 2 is loaded into drive 1 also d 2 1 is read for extra pulses d 3 1 is erased d 4 1 is read for drop outs and d 5 1 is written at 2f as illustrated this process is then repeated as often as desired using 210 ms as the time per track for each of the functions w2f r2f wof and rof the minimum cycle time is 210 ms times the number of tracks 160 or 33 6 seconds for any one of these functions for the entire disk when using an unmodified double sided drive allowing for the additional measurements taa modulation resolution and overwrite measurements a total cycle time for each function can be conservatively set for 40 seconds i e t1 t9 are each 40 seconds long hence the total time for certification of a disk is 200 seconds in this arrangement the actual electronic hardware required for 5 drives is less than that needed for systems using modified double sided drives for example the 5 drive unit requires only one analog board one cpu board a multiplexor board and one digital board whereas a modified double sided drive as used in the prior art requires one equivalent cpu board one equivalent digital board and two equivalent analog boards the cost of the prior art double sided single drive system and the cost of the 5 drive unit of the present invention are substantially equal so that the cost per diskette using the apparatus of.