在U盘上装了个arch，想使用VBox从U盘启动，在网上找到了方法，其实不仅仅是U盘，可以是任何一块硬盘。方法记录在这里

使用VBox的用户需要加入vboxusers组(宿主机)

更改权限

用户需要对要使用的U盘设备文件有读写权限

sudo chmod o+rw /dev/sdx

创建VMDK文件

在帮助VBox帮助文档上找到以下内容

To create an image that represents an entire physical hard disk (which will not contain any actual data, as this will all be stored on the physical disk), on a Linux host, use the command
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk -rawdisk /dev/sdx

执行上边这条命令后，使虚拟机从刚刚创建到vmdk文件启动即可

每次想从U盘启动时都要先执行一次第二步

从硬盘的指定分区启动

帮助文档如下：

To create a special image for raw partition support (which will contain a small amount of data, as already mentioned), on a Linux host, use the command
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk -rawdisk /dev/sda -partitions 1,5
As you can see, the command is identical to the one for “full hard disk” access, except for the additional -partitions parameter. This example would create the image /path/to/file.vmdk (which, again, must be absolute), and partitions 1 and 5 of /dev/sda would be made accessible to the guest.