How to back up again, without losing previously stored info

Hi, I have a WD My Passport Ultra, which I have already backed up documents from my old computer. It still has more back up capacity and I now want to make a back up of my new files on my New MAC, but it is saying it will erase my previous backed up documents. Why can I not add a new back up to the same My Passport Ultra, or can it only be used once? I do not want to lose everything I have previously backed up, as it is different from what I am trying to back up this time. Please help. Thanks.


What was the OS on the old computer? If it was a windows computer, you might have an issue since the files system is different on windows and Mac.

If Both systems are Macs, how are you trying to backup the files?