Hi
It could be plate lift. The aluminium caliper corrodes against the steel pad plate. This causes the plates to lift. This causes a squeezing of the pads, preventing them to move freely. The inner side of the caliper seems more prone to pad lift. This results in the outer pad doing all the work and wearing at a much faster rate than the inner one.
This is what plate lift looks like.
The plate looks as if it has lifted in the corner away from the caliper.
This is what happens to the inside of the disc if you ignore it.
This is what flat plates look like.
If you can get the screws out that hold the plates in, then it is relatively easy to clean it all up and put back together. If you can't get them out cleanly, it can be painful.
Hope this helps. Good luck.
Regards
Oily