sorry, missed that part abotu formatting it. In order to format the disk (if its being detected in the first place) you need to open up the applications folder and enter the utilities folder, and inside is the disk utility. Run the app, then select initialize, if there is an option to do this to the USB disk, go ahead and do it, when it asks you for the format type, select HFS+. If the USB disk doesn't show up, then the computer and drive are obviously not connecting at all, whether from a faulty drive or no driver. DO NOT initialize your 6GB disk.
BTW a drive can show up in disk utility but not show up anywhere else. So dont think that just because you can see it in the finder, that the computer doesn't know its there. The apple system profiler can also help you determine this.