I had the exact same problem as deleon. My new Seagate Freeagent Goflex Portable 1TB kept giving me error after several minutes into the video playing.
Think i found a workaround from another forum. The new Goflex has a power saving sleep mode after 3 minutes. Not sure why, but apparently WiiMC unable to get the drive stop going into sleep mode or simply incompatible with the power saving mode.
Fortunately, the sleep mode can be disabled via Seagate Dashboard. The installer came along with the harddisk.