Time Machine says the back up drive does not have enough space

"Time machine could not complete the backup"

"The backup disk did not have enough space to complete the backup"

I have a 500gb internal ssd in my Macbook with 80gb of data to back up to a 500gb external hard drive, yet Time Machine says there is not enough space on the external drive to perform the first backup.

It is a brand new external drive, with no other data. Can anybody help here please? Mike