fujitsu handydrive Ext HD - communication is cut by higher volume transfers

Good afternoon, my problem is as follows, soes anyone have an
explanation and a solution?

PROBLEM
The 80GB Handydrive shall communicate up to 60MB/s inder USB2.0. By
smaller data volumes per command, copies work fine. When I order a copy
of high volume of data (huge files or folders with much contents), more
often than not I loose control of the Unit, the HD disappears from the
Explorer window, and a message appears that the HD has exceeded the
capacity of the port. I am able to solve it by copying smaller volumes
per command, some may have to be done repeatedly.

In addition, in a backup program (which never works because it moves
mass volumes) I can see that the transfer speed is around 200MB/minute,
fluctuating between 20 and 750. This is certainly very much less than
the announced 60MB/second, althought I am not sure how they distinguish
Bytes and Bits.

I have checked, and seen that appearently the HD is located under the
USB2.0 enhanced brach controller, but still feel that

1. the speed is not at all what it should be
2. the connection is broken if the HD moves faster than the port.

Can anyone explain this to me, from own experience?

Thanks. Knut