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谢谢onshoestring，也谢谢帖子http://forum.ubuntu.org.cn/viewtopic.php?f=77&t=190435 解决方案的原作者billbear。
问题的原因是：虽然安装windows7使用了MBR分区，但是仍然有GPT 分区表信息存在，sudo parted /dev/sda print 中会显示：
Warning: /dev/sda contains GPT signatures, indicating that it has a GPT table.
However, it does not have a valid fake msdos partition table, as it should.
Perhaps it was corrupted -- possibly by a program that doesn't understand GPT
partition tables. Or perhaps you deleted the GPT table, and are now using an
msdos partition table. Is this a GPT partition table?
Yes/No? Yes
解决方法是，用live CD引导进入Ubuntu,利用如下的命令清除 GPT signature
sudo dd if=/dev/zero of=/dev/sda bs=1 count=8 seek=512
问题解决了，分区配置时可以看到NTFS分区了。