My old 360 60gb hard drive, plugged into new Xbox (by transfer cable) will not show up when I want to download something (Halo 4 online maps, for example) but will show up when I transfer things. Am I doing something wrong?
You cannot use an HDD Drive from an older system as a save location for a Slim Xbox. On the Microsoft website for the transfer cable:
Note that there is a way to get this to work, but it isn't officially supported by Microsoft. This answer on Yahoo! could be a starting point, and also confirms that you can't use the transfer cable and an old hard drive as a save location.