@Russ - Yes, on this PC with the old .FTW file I opened it with FTM 2005 SE. If I go to my mother's entry (for example), she has 95 media files in there and I am able to view them all while using FTM 2005 SE. But then when I go to export that FTW, bring it over to the latest Mac version, and import it (turning it into a .ftmm2 file with side media folder), and I go view my mom's page, it shows thumbnails for all 95 media files, but I can't open and view most of them. I get that "file not found" sort of popup. If I go and look at the media folder with Finder, there aren't nearly enough media items in there (they're all images by the way, nothing fancy).
Quick summary: I made an updated .FTW file with 2005 SE, brought that over to the Mac, and then opened that file (it crunches it and turns it into the newer .ftmm2 format). And then most media files missing (just thumbnails).
I can't answer exactly how many are missing. I'm not sure how to count all media files with the PC 2005 SE version. I guesstimate probably 7% of the media files are making it through.
Honestly, I'm thinking of acquiring a PC version of FTM 2012, stepping forward to that with the .FTW I have, and then trying to jump it over to Mac.