I had this problem yesterday.
Try this in Terminal Emulator:
mv download download.bak
ln -s /mnt/sdcard/market-download-cache download
You must have root. And Terminal Emulator is an app from the Market, if you don't have it, try sideloading it by finding an .apk from the internet. I don't know of any other way, but there should be a better solution if you can't find it.
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