NF7-S, external USB hdd disappears

Does anybody having resolved the problem, that an external USB-hdd
disappears after a while?

I have the problem, that the partitions are available in the explorer,
but after a while, they are not available any more. When I go to the
administration tool in the system settings, it is not possible to have
access for formatting because the drive does not exist. Sometimes, if I
switch of the power supply of the external hdd, it appears again, but
not each time.

On another pc with Intel Chipset and Pentium CPU it works flawlessly. I
have this effect with to different external hdds. Both supporting USB 2.0.

Who knows a solution? Or is it just the buggy chipset?

Bye
Andreas