They could of averted this by simply asking the user which drive to restore the backup to. I’m a bit surprised Valve would allow such a bug to exist, especially considering how easy it is to encounter. When I ran the “steambackup.exe” from there, it actually worked fine – proving my theory. To get it working, I had to copy the files from the DVD to my hard drive. Considering I had at least 40 gigs free on my drive, I immediately started to suspect that the Steam Backup Tool was trying to install the files on the DVD, which would never work. I executed the “steambackup.exe” file straight off the DVD, and got that error right after clicking “Restore Left 4 Dead backup”.
There is not enough free disk space to run Steam.