I think your SD card got no problem. It is because the iTunes songs are DRM-protected. That is to say, you cannot only play iTunes music on non-Apple platform.
To play the iTunes songs in your new HDD, you should get rid of DRM code with Apple Music Converter at the first place, then copy the...
MP4 is just a video container, it might have different codec. (video codec and audio codec)
If your device can't play the MP4 files you have, you need to convert the MP4 to other format or MP$ (with another codec combination) with Video Converter Ultimate.
Ahhh....I prefer to edit video on PC. So every time I would transfer the video files to my PC first and then edit it on my PC with Video Editor.
By doing this, I can save backup the video files on my PC.
I switch between iPhone 7 and Samsung S7.
The biggest problem is transferring the music I downloaded from Apple Music to the Android device.
I need to convert those DRM-ed Apple Music files to plain MP3 files (with Apple Music Converter) at the first place and then copy them to S7.
No, Samsung cannot play .m4a files natively.
You can convert the .m4a files to .mp3 first (with Apple Music Converter) and then copy the .mp3 to your device.
To move from iPhone from Android is kinda tricky for a music lover like me.
I always got a bunch of music in iTunes which I pay every month...I know other music apps might also be great, but I still want my downloads come with me...
so I find a way out to save apple music in my local folder...