The subject behavior occurs because Plug and Play adds a value to the PnP ID, which will be different if the bus on the client is different from the master. Plug and Play doesn't install the floppy hardware fast enough for the drive to be available when Setup checks it for the Sysprep.inf file.

To workaround this issue, copy the Sysprep.inf file to the Sysprep folder, normally C:\Sysprep.