There’s a funny bug in Lightroom on Windows when you try and select a folder or drive:
When I try to export a selection of images from LR using Specific Folder (or choose folder later) the only available location that shows up in the hierarchy is my Desktop. No other folders/drives are visible. I had this problem once before and managed to resolve it but don’t recall now what I did. I’ve tried shutting down LR and restarting. Rebooting the computer and restoring the default export presets. If I select Export as Catalogue, then my entire folder/drive hierarchy is visible. Using LR 3.5 on Win7 Pro 64.
It is usually triggered when a lot of files are on the Desktop, and you can try this. In the folder dialog, at the bottom you can type “D:” or whatever, and doing so once usually resolves the bug.
See this Adobe knowledge base article.