The Finder sidebar is a convenient location to access frequently used items such as hard drives, various folders in your home directory, network shares, and any custom items you drag there. If you use the sidebar for accessing hard disks, then there may be times when certain hard disks do not show up here, while others do.
The likely reason for this occurring is if you have removed that specific hard drive from the sidebar. Even though the Finder has a sidebar setting you can check or uncheck to globally show or hide all external hard disks, there is a gray area where only some of these enabled items will be shown. This happens when you select and drag items off of your sidebar manually.
If you end up with a drive that does not show in the sidebar while others do, then you have two remedies:
1. Drag it back from the top level of the Finder
You can always open a Finder window and press Shift-Command-C to go to the top level of your computer, where you will see mounted network shares and hard drives. You can then locate the one that is missing from the sidebar and drag it back.
2. Uncheck and recheck the Finder’s sidebar settings
If you have more than one drive that is missing from the Finder sidebar when attached, then instead of attaching each to add it back, go to the Sidebar section of the Finder’s preferences. In here you should see a dash in the checkbox of the “External Hard Disks” setting, indicating it is partially enabled. Click this until there is a check visible, which will have all drives show up properly.
Reset PRAM. That has saved my bacon many times to fix weird issues.
That’s overkill for the issue Topher is describing and probably would not affect it at all. The drives not showing in the sidebar is not a power or hardware issue. And resetting PRAM doesn’t affect most system preference settings, including those in the Finder. Resetting the PRAM has its uses but is not called for in this situation.