I don't know if this program is available for OSX, so give it a try. First open Terminal.app, thenL
$ sudo /sbin/hdparm -tT /dev/hda
hdparm is a command line program to benchmark your disk drives as well as modify settings. You can tweak it for optimal performance but I would imagine OSX does this for you at boot up. the "-tT" parameters instructs the hdparm program to just do a benchmark. The results are outputted as read/write speeds.