I create a single slice and now want FW to slice the rest of the image. I select the image and the slice and right click and choose insert > rectangular slice, though it adds a new slice covering the whole image underneath the current slice? How can I do this?
Drag guidelines over the image where you want the slices to be. In the Export window, choose Slice Along Guides, and check Include Areas without Slices. Note that it is always better to set up slicing manually rather than allowing the software to do it for you.