OK, so *.MER files isn't it. Of course you've rebooted to be sure the OS isn't losing track of available space.
In general, those error messages mean what they say: the onboard storage doesn't have enough room for the *.MER file you are downloading. That storage area holds data logs, runtime files, and KepServer configurations.
The next question is: does your app, or other ones that ran on this machine before, use data logging that might fill up that space ?