This is actually a common problem. I thought I lost one of my 8GB sticks because of that error. Windows somehow locks a drive letter to a particular USB stick. If you remove it at the wrong time it will always give you that error when you put it back.
After a lot of Google searches, I fixed it by going into Administrative Tools-Computer Management-Disk Management. Right click on the USB drive and change the drive letter. That fixed it for me.
There is a more involved fix I searched and found. I pasted the solution here.
Solution to USB or FireWire drive not showing.
However, the disconnect conflicted storage device to free up the drive letter fix may not be practicable or usable by some, such as computers which are on a network or system running data transfer activity to all its drive assignments continuously flow without stopping, and thus disconnect or reassigning any mapped drives, networked drives, removable drives or other storage drives is not an option.
In this situation, there is another workaround hack to fix the no USB or FireWire drive issue. The workaround fix relies on the registry tweak below to change the drive letter that has been previously assigned to the mounted USB or FireWire portable mobile flash or hard disk drive. Or if users wish, can opt to delete any reference to the detected drives by the device so that when the storage device is plugged in again, the whole drive letter assignment will be start from fresh, and clean.
1.Login to Windows as an Administrator.
2.Open Registry Editor by typing regedit (or regedt32.exe in older Windows prior to XP) in Run command of Start Menu and then press Enter.
3.Navigate to the following registry key:
4.Optional step which not required in Windows XP or later, right click on MountedDevices, then select Permissions. You can also click Permissions from the Security menu.
5.Optional step which not required in Windows XP or later, check the option to make sure that Administrators have full control to the registry key. Change and revert back this setting when you are finished with the reset of the steps.
6.If you run above 2 steps in OS earlier than Windows XP, quit regedt32.exe and run regedit.exe.
7.Also in the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices registry branch selected, in the right pane, find and locate a registry value which represents the troubled USB or FireWire drive. The registry key should have the name that resembles the format of \DosDevice\X: where X can be any alphabet letter which corresponding to your physical system drive letter.
Here?s a guidelines that you can follow to quickly determine which registry key is linked to your conflicted drive letter:
Exclude \DosDevice\A: and \DosDevice\B: which normally reserved for floppy disk drives.
Exclude \DosDevice\C: which normally is the system root drive.
If you have more than one fixed hard disk drives, then any drive letters that are using by them is not possible linked again, UNLESS the additional hard disk drive is added AFTER you first use the USB or FireWire device that now unable to show.
Same case with CD-ROM or DVD-ROM optical drive as above reasoning.
If you have inserted and mounted a lot of thumbdrives, USB flask drive, or external hard disks before, you will likely still see a lot of remaining registry values that you won?t know which is which. In this case, double click on each remaining registry key values to view its binary data. Inside the binary data, there will be trace of the name of the device that this registry key represents.