Why are files not displayed on SD card or Is data transfer not possible?

I bought an SD card (16 GB SanDisk) for my HUAWEI P9 lite mini to store music / photos etc. Externally.
My problem is now that it comes again and again to interference with transfers from computer to smartphone:

either files in the folder are not displayed at all
or adding files in the respective folder on the SD card does not work (-> loads continuously)
also files will not be displayed on the smartphone

Through frequent trial and error, etc. It then temporarily works again.

What can this be?

Remove the card and read it with a card reader on the computer.

I've already tried with the card reader on the laptop, but does not work either.