Hello, gaminhk!
If you want to transfer files from a Mac to Windows PC and vice versa using an external portable hard drive then you have to have the external drive formatted in a file system both OS's can read. Windows uses NTFS and Mac OS uses HFS and they're incompatible with each other. However, you can format the drive to work with both Windows and Mac by using the exFAT filesystem.
Thank you for reaching out and using the Apple Support Communities. We see from your post that your seeking a way to make your MP4 videos viewable on both Mac and Windows PCs. Having that shared compatibility between both operating systems is very important, especially when you want to share to them with others.
In this situation, the best way to ensure that both a Mac and a Windows PC can access the videos from the same external device is to make sure the device is in the correct disk format. This article explains how to format a device or disk (e.g. USB flash drive, external hard drive) into the proper format that is accessible by both Mac and Windows computers: Disk Utility 12.x: Format a disk for Windows computers
In regards to your second question, check out this article: If QuickTime Player can‘t open an audio or video file
Please let us know if this resolves your issues.
Kindest Regards.
Sep 1, 2016 10:19 AM