External HDD Unmounted After Sleep
If only we've searched before buying we would find this drive has this problems. It seems it lies either in firmware of enclosure or in firmware of a disk used inside. I didn't find FW updates for it on Samsung support site (but they have a habit to hide such stuff).

Probably, you may upgrade FW for the disk used inside enclosure and it could help but no guarantee.

If you can get your money back just do it because it seems this product is faulty (it probably behaves the same under Windows (you may try it if you could)).

If you can't get your money back or exchange it try this solution from MacOSXHints.