Okay before you reflash can you type this into your Xterminal:

Xterminal:
df -h

show the output here and it really make it easier for one of us here to help you out.

in /var/cache/apt/archives
there are 2 files:
lock and partial

mkdir = make directory, I think you created something and moved something by the look of it.

you might of deleted those files or modified it. That's why you get the error.