GarageBand 10.1 will not open older GarageBand files after installation.

Which version of GarageBand created the older projects? And what exactly happens when you try? Are you getting an error message? If yes, which message?

I just tested, after I saw your post and GarageBand 10.1 opens any of my older Garageband projects created in 2009.

When I try to open the file GarageBand immediately asks me to save a file with the same title with the digit 1 added to the file as a suffix.

I am seeing the same. GarageBand 10 creates a copy with a new name.

If I do that it "opens" the file but the original tracks are gone.

But a original tracks are there for my projects.

Have you been using special tracks with custom instruments? Third-party plug-ins?

Does GarageBand work well, when you try to create a new project?

As test, launch GarageBand and select to create a new project. Add a few tracks. Does that work?

If not, remove all traces of GarageBand from your User Library.

• Delete the folder ~/Library/Containers/com.apple.garageband10/
• Delete anything with com.apple.garageband in its name from ~/Library/Preferences/

Your user library may still be hidden.To reveal the hidden User Library:

• Bring the Finder forward by clicking on the Desktop or the Finder icon in the Dock.
• Select your user/home folder (with the house icon in the Finder sidebar)
• With that Finder window as the front window, press the key combination ⌘J to bring up the View options.
• In the View options panel enable ’Show Library Folder’. That will make your user library folder visible in your Home folder.
• Open your Home folder, then the Library folder, then Containers.
• Move the complete folder ~/Library/Containers/com.apple.garageband10/ to your desktop. Move the entire folder, not just the contents.
• Then open the ~/Library/Preferences/ folder and remove anything with com.apple.garageband in its name from ~/Library/Preferences/

Now restart the Mac and try again. If all is well, delete the folder from the Desktop.