Using Windows 10 and LIghtroom version 4.1: I have been importing RAW & .tiff files from a folder during a poject. During the process of importing the files were removed from the folder one at a time as the importing continued. This has never occurred before and I have no idea why. Any help most welcome.
Assuming you use the ancient Lightroom 4.1 that is now Lightroom Classic, then the answer is simple. At the top of the import dialog, you can choose between 'Add', 'Copy', 'Copy as DNG' and 'Move'. If you just want to add images to the catalog, and keep them in their current folder, then choose 'Add'. It sounds like you chose 'Move', so Lightroom did not only add them to the catalog but also moved them to another location (as defined in that same import dialog). That is why you saw them disappear from their current folder one by one during the import.