I have a similar cheap enclosure which showed similar symptoms over the weekend. FireWire has never worked reliably on it, so I switched to using a well shielded USB cable almost immediately.

My disk appeared to fail yesterday when I had plugged it into my new Mac mini for the first time -- Finder mounted it sometimes but Disk Utility refused to recognise my partitions. Eventually, I spotted that the USB cable was wrapped around an external power brick, and simply relocating it cured my problem immediately.