Hello Nigel,
After reading you reply I got slightlyu worried and rushed off to the Logic Analyser with my one and only disk that I have for the Logic Analyser. This is a 1.44MB.
This one can be read on both the rear and the front drive. Pfieuw.. (I am not completetely oblivious after all .
Back in the Amiga days, my brother got me a load of disks from work, but they were 1.44MB ones, instead of 720KB - I used the "block the hole" trick, and all seemed to work fine - BUT, when you went back a few days later you started getting read errors, and these got worse and worse the longer you left them.
The 720K drive hasn't got enough power to properly magnetise the different material in the 1.44MB disks - so it doesn't write them correctly.
So if you're using 1.44MB disks in 720KB drives, be aware they may now be very reliable.