Question:When I click on the G:\ drive (a USB flash drive). I get the error message "G:\ is not accessible. Access is denied."
Answer: Is it only this USB drive that gives you the error? Have you tried it on another computer? You may want to try the USB ports in the back if you are using the ones in the front or vise versa. Also try booting the computer with the flash drive plugged in. Another option is to try changing its drive letter using Disk Management.
Try booting into Safe Mode and see if you can access it. If so then its most likely something starting with your computer in normal mode. You can use msconfig to disable startup items and troubleshoot it.
If the flash drive uses some sort of autorun feature to start a program when you plug it in then that may be causing the issue. If you can get into it at all then you can look for that program or a file called autorun.inf anddelete it. If you cant see the file then make sure you have show hidden files enabled.
A more drastic approach is to remove your USB controllers from Device Manger, reboot and let your computer find them and reinstall them. The only potential problem would be if Windows couldn't find the drivers for them and you would have to have your motherboard CD or find them online.
If you don't have any important data on the drive then you can right click on it and choose format and see if it is accessible after formatting the drive.
I have also heard of Daemon tools causing this type of problem so if you use it you may want to try uninstalling it.