I wanted to upgrade my iPod Touch--despite it being barely over a year old, and being updated once in that time, the majority of apps I have tried to install in the last few months need a more recent version of the iPod OS.
It needs a Windows (or a Mac) computer to update, with enough space to make a copy of everything on my iPod--in this case, about 12 gigabytes. I have one computer left that can still boot into Windows, and it does have far more than 12 gigabytes free.
Except the iPod gives no choice in where that free space must be, it insists that it must be in the user's "my documents" folder, which is by default on the C drive. Windows makes it slightly difficult to install with 'my documents' anywhere but C, and even more difficult to move them after the install.
When I install Windows, I have been in the habit of partitioning the drive, with a small but adequate partition for the C drive, and the remainder as an E: drive for data. This means that if I have to reinstall, I can wipe Windows without wiping out the data.
There may be a workaround within iTunes, but I couldn't find it in the time I was willing to devote. There is a section within iTunes to enter a path, logic would lead one to believe changing that path would change where the backup would go--but when I changed it to a path with more than enough free space, it still tried to use the "my documents" on C: to put the backup.
I finally managed a workaround by adding a spare old hard drive I had available, shuffling data then using Linux to expand the partition holding the C drive--a project I had been meaning to do anyway. Once I did that, I was able to sync my iPod and run the update. I have no idea how long the update took, but at least several hours with the CPU running at 100% for most of the time, and virtually no indication of progress--usually a symptom of an application that has hung. I left it overnight and when I awoke, the upgrade had completed.
I am still amazed at how amazingly well Apple does some things, and how amazingly badly they do others. My brother is an advanced Mac user. He said that iTunes for Windows left such a bad impression, it kept him from giving Mac a serious chance for about a year.