When I try to upload files in the Media Library, I get the error "failed to write file to disk". But I've verified (repeatedly) that the directory is writable (I did chmod 777 on wp-content as suggested in multiple posts, for good measure). I've even confirmed that WP can write to files in wp-content, because I can use the Theme editor.
So my next point of speculation is: do files get placed in a temp folder first, before being written to the uploads folder? If so, where?
And on a potentially related note, I can't update my plugins either: it keeps saying the plugin contains no files.
My servers are running Apache with PHP 5.3. The entire wordpress site actually resides on an NFS server, so the files are shared between multiple front-end servers.
So any suggestions what might be wrong with my configuration?