You're welcome! Thank you for clarifying, further Mat.
Re: "I copy the file locally , then use trim",
Seems like a jam I get into at times with various apps that need a "Done" or "Apply" to complete an action before I can save.
Wondering if you press "Command + S" (the save shortcut), if you'll get some indication (dialog box?) why save can't be performed at the time. .... ( Eg: QuickTime, may need "Trim" clicked to complete the trim edit, before you can save.)
Else sometimes, when things seem halted in an app, seeming like an impasse, will swipe to another screen, do something there, swipe back and the problem is inexplicably "gone".
Or if QuickTime app is actually frozen, seems the only way is to quit and re-open the app, losing any unsaved portion of edit.
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Re: "I record movies on my video camera in AVCD"
"can't save as the time goes from 1 hour up to 100's of hours in a minute".
"also can't watch them".
Wondering what format (eg ,mp4, m4v, .DV) the movies are in when imported to your MacBook Air / MacBook Pro
For the best playing / viewing with QuickTime PlAyer you may need to convert the files to .mp4, .m4v, or .mov
.mov files are QuickTime Player's "native" format.
QuickTime can play Motion JPEG-based AVIs created by many digital cameras.
Other playable formats are: MPEG-2, MPEG-1, 3GPP, 3GPP2, AVCHD, AVI (Motion JPEG only), DV
Seems, if a file is an older or third-party (non-Apple) media format, QuickTime Player may convert it before playing.
NB: In some cases, a third-party app may be needed to convert files for playing / viewing with QuickTime
Supported Codecs include H.264, H.263, H.261
(NB: The Photos app can also play videos imported from a camera.).
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If you can not save a video, try choosing File > Export As, then choose an option from the Export menu.
(Seems QuickTime Player can play .mp4 files, but doesn’t export as mp4.)
or
When you "share" your movie, QuickTime Player creates a version of the movie that’s optimised for how you choose to share.
ie: Choose File > Share, then choose how you want to share your movie. (Eg: You could use "share" to save for yourself)
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Re: "On my MacBook Air from 2012 I have no issues at all on exactly the same files"
If you are using the same QuickTime 10.5 (1126.4.1.) version, and Mac OS version on both Macs it is indeed curious.
If needed:
Apple has a QuickTime User Guide for Monterey online:
Click "Table of Contents" at the top of the page that opens from the link to see topics / how-to articles
https://support.apple.com/en-au/guide/quicktime-player/welcome/10.5/mac
