M3 Sakura Firmware Download
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http://www.ndscardshop.co.uk/page2.htm
http://www.r4gift.net/
http://www.buyn5.com/shop/index.php
I have used all of the above sites with no problems
Just be aware that the "value packs" and "basic packs" do not contain all items like sd card reader firmware disc etc
Have you tried the latest firmware already ? the problem you described seems result from failed updating . You can just plug in the "programming cable" into your pc,after you download the latest firmware and unzip it,open the folder,copy the root files into your micro sd card,insert it into your cable,and wait for about 30sec.