Ubuntu系统硬盘安装法分盘出错？？？

我采用 grub4dos安装Ubuntu系统时（组双系统）是硬盘安装法
分盘是产生下述错误
提示：the installer has detected that the following disks have mounted partitions
/dev/sda
do you want the installer to try to unmount the partitions on these disks before continuing?if you leave them mounted,you will not be able to create,delete,or resize partition on these disks.but you may be able to install to existing partition there.
翻译：安装程序检测到下面的磁盘分区的挂载
/开发/ SDA的
你想安装尝试卸载，然后继续对这些磁盘的分区？如果您将其安装，你将无法创建，删除，或在这些磁盘.但是你可以安装到现有的分区大小的分区那里。
为什么？帮下，谢谢。