Do you want to hide the disconnected volumes in the left-hand Folders panel, or do you want to hide the photos on those volumes in Library grid view? To reduce the scrolling in the Folders panel, click the black triangle to the left of the volume name to expand or collapse the volume's folders:
If you want to hide the photos in Library grid view, let me know, and I can provide a good way of coping with that (the instructions are a little more involved).
I meant to hide the actual volumes from library view unless that drive is connected. with the more involved instructions Thanks dude
Follow steps 1-3 described in this post to show the root folder for each drive. When you're done, each drive in the left-hand Folders panel should look like:
On Windows, the root folder will be labeled with the drive letter instead of "/".
Now instead of selecting All Photographs in the sources panel, you can select the root folders of just those drives you want to work with. This approach isn't perfect (for example, if you rely extensively on collections, since LR 4 you can't easily take the intersection of folders and collections to show just those photographs in collections that are on the selected drives). But it may make it easier for you.