I don't think there is anything wrong with your Yosemite installation.
You just need to tweak some settings to make your work environment match the one you are used to.
To show the drives on your desktop: Finder->Preferences->Show these items on the desktop: and tick the box for hard drives.
(this used to be the default, but it was changed a while ago, certainly a lot before Yosemite!).
How is your Drobo connected? I never had one, but I gather it could be directly connected or attached to your network. The answer depends on this.
You probably need to redownload GarageBand from the App Store.
Not sure what you call "taskbar"... but you can add things to the Dock, window toolbar, window sidebar just as you could before. No change there.