Wow that was a fast reply; Thank You!
Short answer - I found the file and deleted it, now it works - thank you!
Long answer - I took your advice and went into /var/log. I had a hard time finguring out how to display files by size, but I noticed that my inittab file contained the line
...and so I looked and found a file called mgetty.log.ttyS0, which was 1.8GB big! I deleted this file and now I can copy files, run samba, etc. I don't yet understand why this file was logging various activities about every 15 seconds since I brought the server up (July 2001), but I can certainly live without it.
Thank you very much!