USB HDD is stop working during copy

My External HDD (vosonic vp8390) stop working during coping files.
It still shows in the My computer but EMPTY (can open it but there is no folders).
It shows in disk management as RAW disk.( like it is new and unformatted)
and the most weird thing is - it will still show that way if i UNPLUG it! (until restart of the PC)

this ex-HDD was (and is) working fine in Vista using the same usb cable and slots (2 of them, for additional power. But it is working even with one slot, just not recharging fast enough. recharging - because it is also a media-player).
after restart - all is fine until i try to move some files to it again. (it is not 100% reproducible, it happens 3 times out of..may be 10)

Windows 7 RTM
phenom II 940, Asus m3n-HD-HDMI, 8gb 800(CL4) Kindston, GTX260, seagates (.10, .11)

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