I have two 128GB SD cards in my D750 digital camera.
I had 64GB in my Df camera, so I thought that two 128GB cards would be enough media space for the D750. I set the D750 to save two copies of each photo, one in each SD card. We went on a two-week photo trip and I kept photos I had already taken in the SD cards, thinking there would be enough space to add photos on the trip. I was very surprised when I got the message "The media is full" during the trip.
We were out of the hotel and driving around at the time, so I erased the data on the second SD card to keep using the camera.
When we got back to the hotel I copied the first SD media to an external HD. I have copies on a home PC of the photos I took before this trip, I am trying to keep at least two (ideally three) copies of every photo, even if I have to use both SD cards to store photos on this trip.
I shouldn't forget to check the empty storage space on the media before I go for a holiday.