copying mp4 footage

When copying mp4 footage through MacBook Air 2013 - SD card to external hard disk, the files are jerky and sound disappears.
How can this be avoided?

Just copying from SD to MacBook to ext HDD? No processing?

Are you just copying in Finder or using a particular tool or tools?

Thx for your reply and interest - yes - no processing. and playback uses Quicktime player, or VLC for those files with m4v endings.
What processing should I have done? The files are recorded by the Panasonic HD camcorder in iFrame format, which apparently the Mac prefers.
Could it be better to import into iMovie first?

If all you’re doing is copying the file in Finder then nothing is happening to change file.so it’s more likely the medium.

Here’s the test:
Play the file from the SD card
Copy to your Mac and
Play the file from your Mac
Copy the file to your external
Play the file from the external

In each instance use QuickTime and VLC

If you can post back the results we’ll have a clear idea of the point of failure

mp4 faults

thx for your reply - I remembered that my laptop had a fall about a few days before from the table to the hard wooden floor, and this might have caused the video faults to be showing up, because the faults were not there before. So I am going to get the Air checked out by Apple, and at the same time, take the external HD to the Apple store so that they can run the files there. If the faults go away with their laptop, then my Air needs a repair. Otherwise there does not appear to be any graphic fault with the Air, i.e. steady display etc.

It’s certainly possible, but unlikely with a solid state drive. But definitely worth getting checked.