技术解析
Windows 8.1 64 位企业版宿主机上运行 VirtualBox 5.0.10 ，虚拟机是 Debian 7 amd64
虚拟机去年安装的，天天在用，一直好好的，今天下午 putty 连上去也一直正常，要下班时想退出 putty ，运行 exit 提示找不到命令，然后发现所有命令都找不到了，打开 VirtualBox 上看， mount 发现挂载成只读
强制重启，进入 grub rescue 模式 显示
GRUB loading.
Welcome to GRUB!
error: out of partition.
Entering rescue mode...
# 根据 google 到的信息输入
grub rescue> ls
(hd0) (hd0,msdos5) (hd0,msdos1)
grub rescue> ls (hd0,msdos1)/
./ ../ lost+found/ var/ etc/ media/ vmlinuz sbin/ usr/ lib/ lib64/ selinux/ bin/ boot/
proc/ mnt/ tmp/ dev/ home/ root/ sys/ run/ srv/ opt/ initrd.img keybase/ lib32/ core
grub rescue> ls (hd0,msdos1)/boot/
error: out of partition.
grub rescue> ls (hd0)/
error: unknown filesystem
grub rescue> ls (hd0,msdos5)/
error: unknown filesystem
grub rescue> set root=(hd0,msdos1)
grub rescue> set prefix=(hd0,msdos1)/boot/grub
grub rescue> insmod normal
error: out of partition.
grub rescue> normal
Unknow command 'normal'
grub rescue>
使用从官网下载的 debian-7.11.0-amd64-netinst.iso 启动到 rescue mode ，但在最后选择挂载系统的时候就出错了，不管选择哪个都提示
An error occurred while mounting the device you entered for your root file system (/dev/sda1) on /target
Please check the syslog for more information.
选择不挂载分区进入 rescue mode 后，/var/log/syslog 信息如下
rescue-mode: partitions found: /dev/sda1, /dev/sda2, /dev/sda5
rescue-mode: selected root device '/dev/sda1'
rescue: umount: can't umount /target: Invalid argument
kernel: EXT2-fs (sda1): error: couldn't mount because of unsupported optional features (244)
kernel: EXT3-fs (sda1): error: couldn't mount because of unsupported optional features (240)
kernel: EXT4-fs (sda1): ext4_check_descriptors: Block bitmap for group 128 not in group (block 811458560)!
kernel: EXT4-fs (sda1): group descriptors corrupted!
kernel: FAT-fs (sda1): utf8 is not a recommented IO charset for FAT filesystems, filesystem will be case sensitive!
rescue: mount: mounting /dev/sda1 on /target failed: Invalid argument
rescue-mode: mount '/dev/sda1' /target failed
kernel: atkbd serio0: Spurious NAK on isa0060/serio0. Some program migh be trying access hardware directly.
rescue-mode: partitions found: /dev/sda1, /dev/sda2, /dev/sda5
rescue-mode: selected root device '/dev/sda2'
rescue: umount: can't umount /target: Invalid argument
kernel: isofs_fill_super: bread failed, dev=sda2, iso_blknum=16, block=32
kernel: EXT2-fs (sda2): error: unable to read superblock
kernel: EXT3-fs (sda2): error: unable to read superblock
kernel: EXT4-fs (sda2): unable to read superblock
kernel: REISERFS warning (device sda2): sh-2006 read_super_block: bread failed (dev sda2, block 8, size 1024)
kernel: REISERFS warning (device sda2): sh-2006 read_super_block: bread failed (dev sda2, block 64, size 1024)
kernel: FAT-fs (sda2): utf8 is not a recommented IO charset for FAT filesystems, filesystem will be case sensitive!
rescue: mount: mounting /dev/sda2 on /target failed: Invalid argument
rescue-mode: mount '/dev/sda2' /target failed
rescue-mode: partitions found: /dev/sda1, /dev/sda2, /dev/sda5
rescue-mode: selected root device '/dev/sda5'
rescue: umount: can't umount /target: Invalid argument
kernel: FAT-fs (sda5): utf8 is not a recommented IO charset for FAT filesystems, filesystem will be case sensitive!
rescue: mount: mounting /dev/sda5 on /target failed: Invalid argument
rescue-mode: mount '/dev/sda5' /target failed
系统可以不要，代码可都在这机器里啊...虽然大部分都有 push 到 git 上，但也还有很多配置都没导出来啊 TAT
现在尝试在 rescue 模式下重装 grub 试试...