The User's Guide for the Commodore 1541 disk drive includes a vague warning, on page 7, to "remember to always remove the diskette before the drive is turned off or on". Some third-party books and magazines issue the same warning even more forcefully—for example, Jim Strasma, writing in the June 1984 issue of RUN, goes so far as to claim that "if you turn the drive on or off with a disk in place and the door latched, you will probably damage the data on the disk".
Now, I had a Commodore 1541 and I must have turned the drive on or off with a disk inside thousands of times. This never resulted in any obvious damage to the physical disk or to its digital contents. Was I just lucky, or is there really some mechanism by which powering up or down the drive might damage a disk left inside? If so, what exactly is this mechanism, and under what sort of conditions might it be triggered?
Note that I'm not talking about turning off the drive while it has a file open for writing; obviously that can result in things like splat files and corrupted block allocation maps. I'm asking whether it can be dangerous to shut off the drive when no disk activity is ongoing.