MOV files not downloading properly

I’m having a bit of an issue downloading MOV files from my cameras SD card on to my Mabook’s hard drive. Basically it says that the files have all downloaded (the transfer bar has gone) but then when I look in the folder I created some of the MOV files have an ‘X’ on them and don’t display a preview. This would sort of suggest they have not downloaded properly or are still downloading. If I double click them they will play but extremely slowly and not watchable. I have tried downloading my SD card on to both my internal hard drive and also an external hard drive but I’m getting the same thing. I’m using a MacBook Pro 16 by the way

Any ideas? Many thanks

[Image Edited by Moderator to Remove Personal Information]