Why not ask Google if other people have seen it? Search for
“ff_audiofile.cpp” assert “Pro Tools”
And the first search result you get back is
http://duc.avid.com/archive/index.php/t-379471.html
So start by trying to trash prefs then read all the other ideas there. Especially make sure you are working on a supported file system type.
If still stuck - How many different disks and partitions do you have connected to your computer, what is the filesystem on each of them? And what audio content is stored where, especially the stuff you are trying to drag and drop and the session? I would also try assuming the session is corrupt and import the current content into a new session, if only for a quick test.
And this is one of the more unusual Assert failures in Pro Tools where there seems to be lots of different claims of causes and cures, so other search results may be worth looking through.