WD 4TB not receiving files

Hi I just received two Mypassport external storage [4TB and 1 TB]. I have a number of smaller storage My passports . I am trying to copy all the data on these small 250 gb drives onto the 4tb drive so as to have all my photos on one backup.

Nomrally I simply copy and paste the data from one drive to another but My passport 4tb will not accept the PASte function. How am I supposed to copy files onto the external storage? I have never had this issue with any WD drive before.

Thank you