HD floppy hw solution?

[psxphill]

Has there ever been a gadget that interfaces between an hd floppy drive and a standard Amiga and does MFM en/decoding on the fly?

It would be easier to create something that buffers an entire track. For reading it's relatively easy. You read from the disk and send it to the Amiga at half speed.
 
For writing you have to hold off stepping the head until the data has actually been written. Although you could start writing to the disk after you have received more than half a track, which would reduce the amount of extra delay. You need to avoid a buffer underrun at all costs though.
 
It probably wouldn't be that hard to do, but involves quite a bit of investment which you may not recover due to the small number you'd sell.

[psxphill]

The (very minor) caveat with the Power Computing drive is that it requires a software patch to be able to write to HD disks. Reading HD disks and reading and writing DD disks works fine without it.
 
There are some similar add-on boards that plug into the back of PC floppy drives that make them Amiga-compatible (and I don't think they require a patch). Amtrade made one years ago. There were one or two homebrew boards made more recently, but they weren't widely distributed. Amiga Future wrote about one of them.

The amtrade one was based on a dual speed drive, if you can get one of those then yes it's trivial.
 
The Power Computing drive needed a patch because it needed to wait for the write buffer to empty before stepping to the next track or turning off the motor. You could do that in hardware though.