This is mostly for the slot-less Apple IIc, but it will work (slowly) on a IIgs: http://pcedric3.free.fr/SmartportVHD/
The Smartport interface is no barn burner in terms of performance. I have the Unidisk 3.5" to compare with the "dumb" Apple 3.5" drive and having the IWM directly control the drive is a heck of a lot faster. The fastest 3.5" controller by Apple had to have been the FDHD controller. It was more powerful than an Apple IIe! It reads disks about the same speeds as a modern Macintosh.
The Smartport interface is no barn burner in terms of performance. I have the Unidisk 3.5" to compare with the "dumb" Apple 3.5" drive and having the IWM directly control the drive is a heck of a lot faster. The fastest 3.5" controller by Apple had to have been the FDHD controller. It was more powerful than an Apple IIe! It reads disks about the same speeds as a modern Macintosh.