;-) Off the top of my head, it occurs to me that writing to the diskette requires more power than reading. If your power supply is taxed just to the edge of its capacity… But that’s unlikely. More likely is that has to do with the power cable or connector. Check that they’re clean, etc. Also, try using another power lead, if there is one.
Lastly, perhaps it is to do with the data ribbon cable to the controller. If one of its leads has a break in it, there might be symptoms like that.
:-( After that, I’m tapped out.
