IPhone 3GS Update to iOS 4 and Restore Failure Solution

Situation
Itunes prompted me to download the new update OS4 I did it failed and then the phone went into recovery mode. On attempting to recover it - it failed with a error code 21. Repeated attempts failed.

Tested
Some users report deleting the downloaded file and trying to download it resolves the issue - it did not work for me despite numerous attempts.


Solution

I tracked down a downloadable version of iOS4. (checking the file name matched the file name of the data file in the below location)

Once downloaded I put the file in
C:\Users\Paul\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates
(Windows 7 location - delete the file if there is one already there)

I then opened iTunes and again got the "iPhone in Recovery Mode" and upon starting the recovery mode it loaded the file above and I got a phone that was alive and back to factory settings.

I was then prompted if I wanted to restore from a previous backup, which I did. Once completed the phone was back to normal.