After updating OSX several times, I noticed that the hard drive is working even when I'm not doing anything. Also, the hard drive will momentary lock up the system for no apparent reason. How can I find out what is accessing the hard drive and causing this?
The hard drive may be working due to Spotlight indexing or other background processes. It's not much to worry about normally, unless there are processes gone awry.
As for the system locking up 'for no reason', this most often happens when the OS swaps out memory to disk. You could use iStat Pro to see if this happen, by seeing if your lockups correspond to Swap growing in size.
HDD failure is also a possibility, and as @NoCarrier mentioned, grab SMARTReporter and see if you get any errors.
I none of the above seem to help, open up Console.app (Applications > Utilities) and take a look, or paste here, some logs showing up around the time your system locks up.
its possible that your hard drive could be dying a slow death. Sectors could be failing and the drive ends up thrashing a bit. Try a SMART monitoring tool like this one: http://download.cnet.com/SMARTReporter/3000-2086_4-49263.html to see if there are any problems with your drive.