Ok. ACDSee also does as Lightroom. It creates its own, hdd-based DB after collecting data from the actual photo files; it writes either on DB only (rating, and others), or in sidecar files, or in the jpg files themselves depending on the cases.
There is a quite explanatory paragraph in the on-line help.

It actually is really good in letting you decide where in your file system you want the DB to be, and provides, on its website, a nice video tutorial on how to use the built-in migration feature to move the DB from one PC to another. However, because it is so nicely configurable, one could simply move the files and have the newly-installed software point at the selected db location.

ACDSee is proving, in my little test in these days, a really good software. The GUI is not as nice as Lightroom, but it is indeed very good and more configurable. It is not as feature rich in terms of editing, but very rich nonetheless. And it is faster, by far, than Lightroom.

Plus, it comes with very nice features Lightroom does not offer: a blazing fast, flexible and poweful quick viewer, and, also important, it automatically detects new dirs and files added to your collection. Lightroom does not, and when you force it to do so, it has been incredibly slow in adding pictures and metadata.

So, ACDSee has all I seem to need (except for face recognition, which I gave up on), and one show-stopper : it won't read Canon Lens Used metadata. It is being requested by dozens of users (some professionals among them, I read) on their forums, and there's no word from the producers. That's quite disappoiinting. I would be seriously considering to purchase it, but it is annoying for me not be able to sort pictures by lens used. Too annoying, I am afraid, to justify $180.
Yes, I may do that from windows explorer, but I'd rather have it within the same sw I use to organize and catalogue my collection, of course.

In any case, I'll keep testing... I may change my mind and decide to get it, at the end of the trial period. I still can't find anything else that gets as close to what I want as ACDSee, I have to say. If only lightroom came with a nice quick viewer...