Go into the Passport’s Web interface. In the ‘Admin’ tab, select “Restore Factory Settings”. If you do not have any data on the disk, select 'System and Disk", otherwise choose ‘System Only’.
Also, you can try downloading the firmware update and upgrade using the downloaded file.
Make sure you only have a single partition on the MPW and it is formatted to FAT32, ExFAT (default), NTFS, or HFS., if you reformat it. If you have multiple partitions, it will confuse the MPW firmware and you will not be able to do a number of things, including firmware updates (I tired). What are you formatting the MPW with?
If you have copied a lot of content on the MPW, It might take 30 seconds or so for it to recognize the data on HDD drive and give you the pie chart in the WebUI. If you get the pie chart, with all of the different file types, you should be able to update and not get the message, out of space.
If you still can’t update, contact WD Support. They might have another way of updating the unit.