Probably this, from fs-uae.net:
Empty hard drive images must be partitioned and formatted before use. Workbench may come with a tool called “HDToolBox” which you can use to partition the image. By default, the hard drive is accessed through a HD controller called uaehf.device. You need to tell HDToolBox to use this device when you start it, for example by running the command:
DF0:Tools/HDToolBox uaehf.device
|