Hi,
It's as easy as Legerdemain says... As far as I remember, you also need the "UAESCSI.device" checkbox to be ticked, on the "Misc" page of WinUAE's configuration settings. You also need to go to the "Hard Drives" page and use the "Add Hard Drive" button.
I've used WinUAE to do this process in reverse - and transferred a load of stuff to my A3000's SCSI drive this way. The drive just appears in the emulated Amiga environment and you can use whatever Amiga app you like to manage the transfer, e.g. Directory Opus.
- Ali
