Recently I started getting "IO Error" while backing up my computer that runs under Vista. To back up I used Vista's "Backup & Restore Center". Somewhere closer to the end of the back up process it would just throw this IO error seemingly with no good reason. About five last attempts to backup ended with this error. I used to get that error a long time ago, and then the error would occur only sometimes, and on different systems. That time I was able to eventually track the problem to faulty USB cable, but this time was different - error was occurring on just one machine, and once it started happening - it never went away.
I noticed, however, that on the machine where the error was occurring, system drive where Vista and most of everything else is installed, is running out of free space. I got "only" 1.7 GB left. I didn't think that should be a problem, since 1.7 GB is still quite a bit. Nonetheless, I decided to reshuffle partitions to increase the size of my primary partition. Once I've done that, I made one more attempt to backup my machine, and this time it's worked.
The bottom line: be sure that drive you back up (or maybe just your system drive) has enough free space.
I will update this post if the error resurfaces or if I find out that amount of free disk space has nothing to do with the error.