Did you check you're greeting table times. If you did not change the defaults you may run into problems. What I mean by that is that you  could have a day greeting in a night greeting or the other way around check it out see what you find. You will find the greeting tables in AA. Verify that you're greetings are in the proper time slot

I would assume if you went to the measure of replacing the drive you must have checked the gretting tables and lines settings extensivley. Is this the case? Did you check everyday? Each day of the week has 4 possible settings (morning, afternoon, evening, and non-business). Have all of those have been checked, along with making sure that all lines are being answered by the same table? If this is the case and you replaced the drive, then I would look at the KSU having some type of problem keeping the time, since this is where the NAM gets its time from.