For the FDS to be able to completely rewrite disks and work with the MGD1, the power board needs modified to bypass security features Nintendo designed into the board. Depending on the power board revision in your drive and the drive controller chip it has (FD7201 or FD3206), the process to modify it for full writing ability varies.
A half-modded drive will only rewrite part of the disk and not the HVC-Nintendo header and perhaps some other disk data. Due to the slight variations in drive speed and drive alignment between the original drive that originally wrote the disk and the half-modded drive that wrote the remainder of the disk, the resulting disk copy isn’t perfect and may not work correctly. Furthermore, if the first data block of the disk is damaged, then a half-modded drive won’t be able to rewrite it at all.
FamicomWorld has a great tutorial outlining what power board modifications your FDS will need to enable full writing ability depending on the particular board revision and chip. In addition, they also have a tutorial explaining how to modify a FD3206 Drive Controller Chip board to enable full disk writing ability. For a simple, 8 wire solution, ToToTEK sells a 3206 Add-On Chip to write enable an FDS with a 3206 drive board.