1. Backup what you can't lose first.
2. power off, unplug the cable ends and press them back home.
3. do your yearly cleaning like this -> http://reviews.cnet.com/4520-11319_7-6240575-1.html
4. leave the cover off as some machines overheat.
I have 320 gb western digital hardisk and i newly buy 1 TB western digital hardisk and connect both with my pc some days both works properly but recent days when i open my computer my 320 gb harddisk some time disappears and some time the drive icon seen on my computer and when i click and open the drive icon it hangs and after few second later hardisk icon of 320 disappears but the name is seen in device manager....and when i restart my computer again it shows the drive icon and same problem repeat...i had vista installed in 1 TB harddisk...