The reboots (if you unplug power) might had cause it. For future needs, if you ever need to copy one NAS to another, either use safepoints, or rsync/ftp commands. 400GB should not take 2 days, however, most people will copy the files via the PC/laptop. If you do that, the info will flow as follows:
NAS1 → PC → NAS2, instead of NAS1 → NAS2.
Another option:
To get the data and possible fix MBL, take a look at this guide. Warning: It will void the warranty.
http://community.wd.com/t5/My-Book-Live/GUIDE-How-to-unbrick-a-totally-dead-MBL/m-p/435724
