Move apps to phone. Delete android secure from sd card. Move apps back to sd.

Are you rooted? I never found a solution to low space until I rooted. A2sd and dalvic to cache partition frees up a ton of space....but you can do neither without root and a custom rom

 

Yes, first and foremost, I would suggest you do exactly what Mr. Ed suggested. The directory in question is .android_secure. Note the "." before the name. That means it's hidden. You won't be able to see it in a normal file browser. If you connect your phone to your computer via usb, and mount it as a disk drive, then you'll see it at the top level. Delete it only after moving all apps back to internal storage, as that is where apps (or the portion that is moved, rather) is stored. Likely, there is some corruption in there, especially if you've been unmounting the card, then reinstalling the app. Likely you've got all kinds of junk in there from doing that. One other quick question about this - if you have root, have you forced any apps to the sdcard with the titanium backup?

That being said, your space issues might actually be related to that. It could also be some app that is taking up way more than its fair share of space on internal memory. Quick question, first. If you go to menu->settings-sd & phone storage, what do you see for available space under internal phone storage? If it's very low, then that's a problem. How many apps do you have installed? If it's a lot...well, you may simply have to uninstall a few, or consider some of the root only options that Mr. Ed brought up. Until then, if you have a normal amount of apps installed, but low internal storage, see if you can figure out the culprit. Go to menu->settings->applications->manage applications->all->menu->sort by size. That will sort the list by the amount of internal storage the app is using for both the app itself, and its data. If an app is taking up too much space (likely with data) it will be at the top of the list. Tap on the top few to see how much data they are using. Uninstall apps that are using more than they should be, and you should be able to reclaim all that lost space.