I had the "cannot install on . . ." problem, too. This page Fixing The "Couldn't Install On USB Storage Or SD Card" Problem On Android
helped me. It talks about deleting some stuck invisible files like smdl2tmp1.asec.
The article says you can delete the files using astro, but that's not what I found out. Astro couldn't see them, so I used a USB connection to a PC to find and delete the files.