the only thing is "MHz" which basically means frequency of some components, such as, CPU,RAM,GPU those things have frequency mechanism which could be overclock to get more better performanc, now if we turn to HDD this component does not have frequency it had RPM (revolution per minute) HDD should have fixed number of rpm's some HDD have 5200rpm,7200rpm and 10000rpm those rpm indicates how is your hdd platter speed for example if you have a HDD of 5200rpm this means your hdd platters can rotate 5200 times per minute, the more rpm the more better speed of data transfer so it is impossible to oc it you just think oc is the ssd of this hdd.