I was running Debian Lenny and was upgrading to Squeeze when I got a warning saying "your root partition is now 99% full", whereafter many packages got broken and all kinds of things including GDM melted down. That root partition was of standard size, so why it did this is beyond me.

I have put a desirable live OS on a USB so I can install that to the HD, but suddenly when I boot I cannot get into the pre-GRUB menu where you may select to boot from USB. I suspect my Syslinux is borked, and it tells me "No DEFAULT or UI configuration directive found". I had an Ubuntu CD on hand and installed that hoping that would reconfigure Syslinux, but there is no change. However, I have no blank CDs to write the liveCD for my preferred OS, so want to use USB and want that functionality in perpetuity anyway.

How can I regain the "device to boot from" screen without using a CD?

Thanks for your help.