You have to import photos from your new drive before Lightroom can see it.
LR only lists drives from which photos have been imported.
I guess I wasn't clear...I have imported images from my camera card, through LIghtroom and onto the external hard drive. It's actually more complicated then that. For example I just looked at Lightroom and had my filled external drive connected to work on some photos. Listed under that hard drive for the files that are on it and the files that I imported to the new drive. In addition, LIghtroom had that filled hard drive listed as E: with files and question marks, and "Toshiba" which I had to connect with the missing drive merge. It seems it is having trouble reading the drives. Windows is not, however. When I look at the control panel it clearly and correctly shows the external hard drives that are connected. I have no idea what to do because I have carefully tried to import, use and export...and yet it is getting fills listed all over the place and the hard drives confused. Please advise...I would like to connect directly to an Adobe tech employee but can't find a way to do so.
This looks like a problem with drive letters, and the way Windows assigns them.
Your system hard drive is C, and you probabably have a DVD drive that is D. The next letter is E, which was assigned to your old external drive when you connected it the first time. When that drive is disconnected, Windows considers E not to be in use, and will assign that letter to any drive you now connect, like your new drive.
The solution is to assign a new drive letter to the new drive. I have Windows 7, but it's probably similar in 10:
In Windows Explorer, right click Computer and choose Manage, then Storage > Disk management.
Right click the drive, and choose Change drive letter. It's a good idea to pick a letter at the end of the alphabet, which makes it very unlikely that it will ever be changed.
You will now have to right click the folders in Lightroom that it considers missing, and point to the new location.
Some tips on how to do that can be found here http://www.lightroomqueen.com/articles-page/lightroom-thinks-photos-missing-fix/