I'm surprised to find this crew of iMovie HD loyalists because in OS 10.9.5 Mavericks I've had nothing but grief with this app.
I know a large number of people using iMovie HD 06 successfully with Mavericks and Yosemite. I probably made 100+ movies with iMovie HD and Mavericks and never encountered a problem.
Using Yosemite requires the workaround below.
Yosemite has a bug that incorrectly identifies iMovie 6 (a.k.a. iMovie HD) as not compatible. iMovie HD works fine with Yosemite.
Below is a Yosemite workaround that many use until this bug is fixed.
Find iMovie HD in your applications folder.
Right click on it, and select “Show Package Contents”.
You will then find a folder named "Contents”. Open it. Then open the folder named "MacOS”.
There you will find "iMovie HD". Double-click on this, and iMovie HD will launch.
Please report the bug to Apple, the more reports, the sooner it will get fixed.
https://www.apple.com/feedback/macosx.html
You may be doing things with iMovie HD that I do not. I know many that keep a dedicated computer with Snow Leopard so they can continue to use iMovie HD without risk.