If you don't want others to use your photos and images without your permission, you can create a so called custom watermark with your name or any other text you want.
If you are afraid your friends, kids or any other person you are borrowing your iPad to, might delete an app by mistake or even intentionally, you can enable restrictions on individual all of the apps.
You deleted everything from your USB key or a drive, so there should be enough free space to copy the new files, but you still get the not enough free space message in Mac OS X? This is what you need to do.