As Buraddox says with UnleashX the dash does tell you how much you have free on your main drives (depending on the Skin.xml). It is recommended though that you don't try to fill any drives to full capacity.
For more detailed information rather than use File Explorer it's quickest just to press Y when on a main menu page to bring up Xbox Info. Scroll down to Storage which shows (and makes clear) the amount of free and used space on all drives in convenient table form.
The HDD info under Settings > Storage that you're referring to is exactly the same information but displayed in condensed form. So your 1,234MB/4,321MB is 1,234MB used space and 4,321MB free space left.
I know that somwhere in UnleashX or maybe via the Config.xml or specific Skin.xml it is possible to convert storage space display from MB into GB if preferred.
This post has been edited by fallenangle: Mar 12 2011, 01:09 AM