Symptoms: Google Drive persists in giving the error "Sorry, upload of the file failed". This occurs when using drag-and-drop or the "upload" button in the browser. Errors occur regardless of file size. A good test is to try and upload a tiny text file, to ensure bandwidth or other issues are not the problem. Re-enabling Java in the browser does not help.
More info: Google help had nothing on this error message, and as far as I could see, nobody else had reported this exact problem. I'm guessing people who access Drive via their browser are in the minority.
I could not use the PC Drive interface in this instance since I was uploading video files that I did not want mirrored to my PC SSD.
Solution: You're probably using an older browser. It may have worked fine in the past, but Google is presumably always tweaking / upgrading the Drive interface.
I have stuck with Firefox 14.0.1 up until July 2014 because I knew it and that version was still supported by the Google Toolbar, which I rely upon constantly.
As of July 13, 2014, Firefox 14.0.1 stopped uploading to Google Drive. Or, at least, that is when I noticed the issue. I had not tried uploading for 3-4 months so it could have occurred slightly earlier.
Since Google's first troubleshooting step will be to advise me to upgrade (and/or to use Chrome), I did not bother contacting them regarding the issue.
IE and Firefox 29 both uploaded with no issues, so I used a different PC with Firefox 29 to upload my files. Multiple PCs, actually, since Drive is so slow to upload.
This might prompt me to actually try Chrome.