0 bytes on HD

[Jim Byrnes]

Was doing a backup to a usb drive using Back In Time and came back to 
find a message that I had 0 bytes available.  I didn't realise I was so 
low on space. Looking back I think that despite me using Back In Time on 
a regular basis something changed and it tried to backup to the HD 
itself instead of the attached USB drive.

I deleted a bunch of stuff and emptied the trash.  Now the System 
Monitor says:

Total 91.8 GiB
Free  4.7 GiB
Available 0 bytes

I don't understand why I now have 4.7GiB free but none available.

What is my best course of action?

Would rebooting allow the 4.7GiB to be recognized as available or is a 
reboot just asking for trouble at this stage?

The disk only has 10.04LTS on it and when I set it up I only formatted 
half the disk so I have 100GiB of freespace to expand into.

Would it be best to boot from the install CD  and try to expand it first 
thing?

It's been a long time since I faced a similar problem and hoping someone 
can assist me. Right now the machine is unusable because about the only 
programs it will open a small utility programs.  I had to use my seldom 
used laptop for this message as Thunderbird will not open.

Thanks, Jim