It does work without an SD card, but that's pretty dangerous. The reason being, is you have a much higher chance of wiping your files on your internal SD card, than on your external SD card. There's only 1 way to wipe the external SD card, and that's to select "wipe SD card". Your internal memory, if you wipe your /system or your /emmc or any other file system, you could end up corrupting your internal card, and causing issues later on. To be honest, I haven't seen full confirmation that this will work without the ext-SD file system, but Hash stated that theoretically it should still work. I just wouldn't trust not having a back up on the ext-SD file system.