I don't know wether one is able to change FIFA10s save directory,
but as I think this isn't your main problem, I may still help you :D
The folder Documents isn't a real folder. It's just a view on different folders.
Which means, you can easily alter/add a new location.
(FIFA will still store its files to "documents", but documents is on your other partition)
Create a new folder that you want to assign as My Documents folder,
if you have another drive or partition ex: drive D: I recommended
that you create My Document Folder to that drive.
browse to "C:\Users\" using Windows Explorer (Press WINDOWS KEY + E
to access this). Then, right click on My Documents and select
Properties: Then Click Location Tab
Then click Move then browse to new location for your My Documents
folder. Once you find new folder for your document click Select
then click Apply, there will be pop up message to confirm the
changed, click Yes to move your documents to the new location, or
click No to leave your documents in the original location.
Take a look at this page for any other Windows-Version than Win7.