Yes, but are not going to be able to go very far back in time with only an extra 5 GB. Also, if your main drive is larger than 80 GB then you will not be able to backup in any way once your usage passes the 80 GB mark. If your internal drive is 80 GB and you only have 5 GB free then your machine will start to slow down. OSX needs more than 5 GB free working space.