Sandisk Ixpand 256 go doens't work after 30 seconds


I have an Iphone 6 128 go and I bought a Sandisk Ixpand 256 go.

I charged the usb key with many files and I can see them when I connect it with Iphone.

But when I try to read them or I try to copy a file to Iphone everything stop working.

Can you help me please?

Kind regards.