How do I open old QuickTime movies on macOS Ventura after moving them to an external drive?

This isn’t because Apple stopped supporting QuickTime. QuickTime Player is still built into macOS. The issue is more likely that your 2010 movies use an older codec that the current QuickTime Player no longer supports.

Try this:

1. Check the codec: Select a movie and press Command-I. Look under More Info → Codecs.
2. Try VLC: VLC is free and supports many older video codecs.
3. If VLC plays it: The file is fine. Use HandBrake to convert it to H.264 MP4, which should work with QuickTime Player.
4. If VLC can’t play it: Check the external drive and make sure the files weren’t damaged when they were moved.

Ralph Esposito wrote:

I see that Apple no longer supports Quiktime,

Apple still supports QuickTime. It's an essential component of macOS and it's on every Mac, every version of macOS.

What you have probably encountered is that the "old movies" from your 2010 iMac are probably based on a codec that modern, QuickTime 10 no longer supports.

What error message do you get when you try to open one of them in QuickTime? If you select one of the movie files and then do GetInfo, what does it say the codec & color profile are?

VLC (free download) can open most video files.