Hard drives need to work in a dustless environment, so you CAN take them apart... but don't expect them to work after you put them back together again...
I agree Ive taken apart several but there is not alot you can do with them. Usually when they go bad the motor is the culprit.. Cheaper in the long run to buy a new ,faster HD. R
I have found it is usually easier and cheaper to take it in where there is a closed environment or buy a new one as rob suggested. It never really works the same once taken apart.