Try this

Go to start run type regedit press enter
go to this key HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
right click on the key and check the permissions
administrator and system should have full control if not this may be the problem set the permissions by clicking on advanced then inherit permissions also check the replace permissions on child objects option

Its possible to simply delete the devices registry keys instead of renaming them in the instructions below. But make sure that you do a backup for the registry branch by exporting MountedDevices key, as mistake may cause your system unbootable. The delete all possible USB drives, FireWire drives and external drives which have drive letter temporarily only when inserted in the system, and DO NOT delete any of the fixed hard disk drives or CD/DVD optical drives registry values.

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.

You may also see entries that say \??\Volume{fd2b8768-ac1e-11d9-9d4c-806d6172696f} for example

Heres 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 wont 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. For example, figure below shows a Sandisk Cruzer micro USB flash drive.

With this information, search for a registry value that contains name of your USB or FireWire drive brand and model that having problem showing up in Explorer.

Once the correct registry value is determined (for example, \DosDevices\F:), right click on the registry value name, and the select Rename.

Change the drive letter (in the example is F) to another unused drive letter that hasnt been used by another other drives, then press Enter.
Now insert and plug in the USB or FireWire device into the port again (if its already inserted, take it out and re-insert again). The drive letter for the USB and FireWire should now appear and you can use the drive normally and properly again.

Let me know if this works

I notice you have a C: disk and B: disk
Which drive has been selected for the CD drive?

What shared drives are attached at logon?

This problem is usually caused when a mapped drive uses the next available drive letter, for example E: or F: and that is also the next hardware letter available.
What is the drive letter used for the Large 273GB drive?