Try recording a blank spot. Put your thumb over the camera lens for a bit.

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...I for one, sure would appreciate the return of intelligent conversation, spirit of assistance and the simple yet effective ignoring of those who can't seem to hang with that...
Best regards,
rob

So you got iMovie working now ?
I just started using iMovie - and found a lot of tips at the Apple website. I found out that you can clik at the big T in the center of the screen to add text. Try to drag and drop one of those 'boxes' appearing , in between your clips- without adding text. This should work. If you click that added 'black box' , I think you will see that you are able to determine for how long it should appear !

Other way to do your guitar recording : Find your backing track in GB , make new track ( for vocal or something?) - plug your guitar to your amp- crank it up -and just hit the rec in GB. The built in mic in the mac should catch it ok.
Save the file- I guess into - iTunes ( long while since I used GB....)
Get back to iMovie - open the media-browser ) near the 'T' - find your song and drag/drop in iMovie.
Good luck

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A-man

PS!
Ahrrmmmmm.
Just opened my GB to check it out . This is the older one - '09. There I found an 'eye' down at right side. media- browser. Opened - clicked on films- my iMovies appeared there. Dragged one into the main window , beneath the music tracks , and it appeared in sound-wave -form. I guess you can do a lot from in there I don't have the time or the need to find out more - but I'm sure you can have some fun !

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A-man

Take your mac to an Apple store and sit down with one of the genius Apple reps.
They will show you how Garage Band can be opened up so that instead of working with set parameters, you can actually record in very minute tracks , bounce specific sections on one take to another ....etc...etc...

In other words use GB as if you were in a studio which would cost you thousands of dollars per hour...GB is much more versatile than it appears, it is just set up generically since it was aimed at people who have no clue as to the difference between Flat and Sharp...Let alone Music Composition.

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"C'est parti mon Kiki "

that's pretty much all Apple products in a nut shell. Sorry, couldn't resist, please carry on...

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Please subscribe to my Channel!
https://www.youtube.com/user/b7567

Well if you are looking for max sales and widespread positive response, your product has to be shall we say....Idiot Proof....

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"C'est parti mon Kiki "

Some sec's w freetime at work
Also got a mac here - I think the best solution for you must be using Quicktime. You can record the screen with you on webcam while GB does its job in the background. If you use a mic - don't forget to use headphones

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A-man