Thanks all for the interesting feedback! Mydos seems to have some powerfull options, but what I'm looking for is something similar to "US Copy" by E.Reuss.
Means: can be run from a gamedos, fast loading time, support of extra RAM and easy to use (just set destination drive and start copy).
In my case it should work with my HDI connected to a 3,5" HD floppy drive...