I had this problem. When I connect my Samsung Galaxy S3 to my desktop computer via USB, sometimes they would connect and sometimes not. However, if I connect to my laptop, the phone connected all the time.
I finally fixed this, sort of, for my desktop. It still has trouble connecting sometimes but not as bad as before. This how I did it. I cannot remember exactly the small details but I’ll try to explain it the best I can.
First, check if you have the same problem I had. Connect your phone to the computer via USB. Go to your Device Manager (Start->Computer, right-click, select Manage, and select Device Manager) and check if you see warning label for SAMSUNG Mobile USB Modem and, I think, Standard Modem. If you do, right click one of them and select Properties. In the General tab in the Device status window, check if you see something to the effect that a previous instance of the driver is already installed. If you do, try the following. It might help.
- Disconnect your phone from the computer.
- Uninstall Kies from your desktop.
- Restart your computer. This is important because at restart an app, I think it’s called MASetupCleaner.exe, starts. Allow it to run.
- Go to the Samsung website and download the latest version of Kies.
- Install Kies.
- Run Kies.
- Connect your phone to your computer. Wait a bit while your computer installs the drivers. Afterwards, the phone should connect. Check the Device Manager to see if the warning symbols went away.
The MASetupCleaner.exe is important. If you don’t run it, the re-installation of drivers may not work.
I hope this helps.