It can be quite annoying when you click on the Safely Remove Hardware icon to stop using a USB device, but only get the following error message:
This device is currently in use. Close any programs or windows that might be using the device, and then try again.
Problem is, you have closed all the programs but the USB eject still displays the same error. If you have this problem when using the USB icon on the system tray to eject, it can sometimes be the explorer.exe session holding onto a handle. To solve the problem, all you have to do is follow the following six steps:
1. Make sure you have closed all the programs.
2. Click on "Start", select "Run", and type “taskmgr.exe” or Press Ctrl + Alt + Delete to open Windows Task Manager.
3. Right click on the explorer.exe file and choose “End Process” from the menu. When you see a pop-up notification warning, click “Yes” or “End Process” to close explorer.exe.
4. The desktop will disappear for a while.
5. Click “File” and select “New Task (Run…)”
6. Enter “explorer.exe” and click OK.
Now you’ll be able to safely remove your USB device!
Ps. You can also use free tools such as Unlocker, UnLockme and UnLock It to fix problem ejecting USB mass storage device error.
Source : http://my.transcend-info.com/Press/DrT.asp?LangNo=0&axn=Detail&PrsNo=113&NewsKeyWd