Hardware or Software Problem: Firewire Keeps Disconnecting

Hi. The My Book Studio from Western Digital, external hard drive disconnected but when I reconnect it, it won't mount because it can't even be seen by the iMac on both Windows (Bootcamp) and OS X Mountain Lion 10.8.2. I checked it's own SMART diagnostics tool and it says, it's a pass (I also used it's quick test which also says it's pass). If I connect a USB cable though it works fine but I'd rather use the Firewire.

I got to mount it when I flashed the My Book's firmware to the latest once. After that, that method doesn't work anymore. I resetted the SMC and the the PRAM just now, this one mounted. So far so good. I'm not sure if it'll disconnect and never mount again but could this be a software problem or a hardware problem. Is it the iMac's hardware or the My Book? Is it OS X or the software that My Book uses which is came with this external hardisk.

Thank you in advance.

God bless.