I have a small 12MB .rar file. I didn't have `unrar` installed and I tried to unpack the rar file. The file is password protected. I input the password and Ark gives a "Not enough disk space error". I install `unrar` and everything works again. I did not get the "Unable to unpack *.rar file, rar component not installed error". It seems Ark has a tough time with the rar plugin as numerous other errors like such exist.
(See also Bug 454320 Comment 2)