[ericob]

It is more the potential for irritation than concern for harm (although there is that possibility too). Perhaps I'm being superstitious, but (during a restore) I'd just as soon not give the OS and any of it's "automatic" systems something else to play with. I realize the image will be mounted only for a little while, but it irritates me to think that Spotlight (or whatever) might start (pointlessly) scanning it.

As to harm, there is a potential for harm because the mounted image is writeable. Of course the image was created read/write because, well... you couldn't very well back up to it if were read only!

However, when "ejected" any changes, whether accomplish by me (by mistake) or some mystery or rogue process will be, without any notification of course, written to the image file! While this is unlikely to happen, there is nothing to prevent if from happening. Unlike if the backup files were in an archive or an unmounted image file.

One can "convert" a read/write sparse image file to a read only .dmg file, but this is yet another step, and something one must remember to do. Irritatingly, the conversion isn't just a switch you can flip to lock the existing image... an entirely new one is written.