External HDD Unmounted After Sleep

Question

I have the new iMac 21,5" and a Samsung G3 Station 1,5TB external HDD. When I plug my HDD, connecting to energy and via USB, no problems, Mac OS recognizes it and I can use it normally. The problem appears when I put my computer to sleep. When it wakes up, my HDD was unmounted and to use it again I have to remove USB cable and reinsert it. This is annoying me. I have looking for suggestions or improvements to put it work better, but no idea. If you have any ideas, please let me know.

Additional Information:

1. Snow Leopard 10.6.8;
2. On restart it works;
3. Disk utility cannot see HDD after it wakes up;
4. My HDD is formatted on NTFS. I don't know exactly, but it could be part of the issue. Unfortunatelly I can't format it to HFS+ or FAT32. My files and backup are all there.