Using the example of K:(H:), the first part outside the parentheses is the volume label. You can rename this at will by right clicking, selecting Properties and entering whatever you like (e.g. MyDisk) in the box on the General tab.
The second part inside the parentheses is the drive letter. Normally these values are quite sticky within Windows (in that once you have assigned a letter it stays with the letter). But external devices can be redetected and end up with different letters due to a variety of reasons (e.g. using a differnt USB port, having other USB devices plugged in at the same time).