I imaged a disk twice, and they produce very different files! Is this normal?

It is completely normal that imaging a disk twice will produce very different files. The chances of any track being the same is far less likely than winning any lottery.

There are two reasons for this:

1) We measure the density, that is, the concentration of bits across a Track, and we get a reading for each bit. The fact that the disk drive is a mechanical device means that this will never be the same twice, it is an analogue measurement.

2) Some tracks (or partial tracks) may be unformatted, this is usual on cylinder 80 and above, and is also occasionally used as protection. This means that all that is read is “noise”, i.e. random data. This is because the magnetic properties of the disk cannot be interpreted by the floppy drive, since it cannot determine if [glossary:bit cell|bits in each “cell”]] are 0 or 1. They can be either, and different on successive reads. Some copy protections actually take advantage of this fact, since when a disk is copied the bits no longer change because they were written as they were read that particular time.

The same reasons apply to different size dumps. Since we use a type of (lossless) compression to reduce the size of the density data 1). This means that small changes can result in drastically different data and data size for different reads of the same track.