Catalog previews not always generated.

Hi - I seem to have an ongoing problem that drives me nuts!!  I have my images on an external hard drive in sessions and all the subfolders.  When I import a session into my catalog using the "File - Import Session" , the file structure is there along with about 60% of my image previews.  The other previews are not there.  I can view them of course by plugging the hard drive in.  But after the import has completed and I unplug the hard drive some off the previews are missing.  This has happened ever since I have used Capture One.  does anyone else have this issue?