There is a known boot-of-death issue with the 'cuda 7200 RPM drives. At one time, seagate was offering to repair for free. Don't know if that is still the case. There are links to further information, as well as a URL that you can use to enter serial number of your disk and see if it is one that has the problem.
There is no way to fix the problem after the fact, it has to be sent to a data recovery firm (or obviously seagate since it used to be free - perhaps still is). Note, if you do have a 'cuda 7200 you need to update firmware BEFORE it manifests itself, so while you are at the site, download the software so you can flash upgrade the other disks.
Link to the boot-of-death info
http://storagesecrets.org/tag/boot-death/This *COULD* be the boot-of-death problem come to think of it, assuming problem isn't silly like you programmed the disk to go to sleep after 30 secs, which I do not think is even possible to sleep such a short period of time.