OS X V10.7 Lion :: Missing Disk Space And No Usb Startup

I have a white macBook (10.7.3, 250g HD) which has begun to behave in an odd way and which I have so far failed to resolve. 

First I got a message saying very little disk space left - only 420MB which is strange because I knew the HD to be far from full. The beach ball gave me very little access to the Finder but I was eventually able to empty safari cache which gave me 6Gb.  

Checking the root disk folder sizes showed that only 130gb was listed in the various folders but the status bar showed that only 6gb out of 250gb was available. I tried to start up from an external USB disk (no Firewire on this machine, sadly) but the laptop refused to starup from it - tried cable in both USB ports, disk is GUID formatted. I shut down the computer - had to force shutdown with power key as unresponsive to Finder command. 

On restarting I got a message about forced shutdown etc but status bar now says 40gb available (when there should be over 100gb). I was able to select external USB drive in Startup preferences but machine will not startup ffrom external drive. 

Disk Utility reports no problems with internal drive. I want to use Disk Warrior but cannot find cd, hence attempt to startup from external drive. Cannot even check disk while running DW from internal disk as it says prefs file corrupted or write protected. Trashing it (as suggested) makes no difference as new prefs file still unusable. 

I have a TM backup but don't really want to use it if it is likely to have the same corruption.Also, why is the USB disk not starting up the Machine? Only thing I can think of is to do an archive/install of Lion, but is the bloating in the system or are ther some hidden files somewhere which won't be changed?

Info:
MacBook, Mac OS X (10.7.3)