The cause of “grub rescue unknown file system”
The reason for this error can be that you performed abnormal deletion of a partition where second/dual OS is installed. For example, this could be a PC where you were dual booting between Windows and Linux. However, you deleted the Linux partition. When you’re setting up such a system, every partition has an MBR, which has important data on how to boot your operating systems. After you delete the partition or OS, the MBR still has old boot sector values for either operating system and booting up may give you an error, for example, “grub rescue unknown file system”.
What to do if grub rescue file system is unknown?
If you encounter the issue, don’t be panic for there are several solutions to it.
Method 1. Use a Windows bootable USB stick
This part of the content is going to outline the method on a Windows 10 system, but it’s very similar for Windows 7 and 8 too. First, you need a Windows 10 installation CD/DVD or bootable USB drive.
1. Boot into Windows 10 from the installation media and select the option “Repair your computer”.
2. Hit the tab of “Troubleshoot" and select “System Restore” to restore it to the restore point you set up before.
3. You can also try the option of “Automatic Repair”.
4. If it doesn’t work, you need to go with the option “Command Prompt”.
5. Then, enter the following commands into Command Prompt one by one to rebuild MBR.
After executing the commands above, you should be able to successfully boot into your computer. However, if the issue is still there, you might as well jump to method 3 to try third-party freeware to rebuild the disk MBR.
Method 2. Enter grub mode to fix grub
For the error “unknown file system grub rescue”, one of the most common ways is to use command line.
1. You have to find out which partition is your Linux in and where the grub directory is. It’d be better if you still remember, but it doesn’t matter if you forgot.
2. Type “grub rescue>ls” and hit “ENTER” to list all disks and partitions. For example, hd0,(hd0, 1)//,(hd0, 4),(hd0,7),(hd0,8),(hd0,9)…
3. Type “grub rescue>ls (hd0,0)/grub and hit “ENTER”. Repeat this step and try every partition one by one until the contents of the partition appear instead of “unknown filesystem”.
4. Let’s suppose that when it’s (hd0,8), the contents successfully appear.
5. Type “grub rescue>set root=(hd0,8) and hit “ENTER”.
6. Type “grub rescue>set prefix=(0,8),grub” and hit “ENTER”.
7. Type “grub rescue>insmod/grub/normal.mod” and hit “ENTER”. Now, you’ve quit grub rescue mode and entered grub mod which you should be familiar with.
8. Type “grub>normal” and hit “ENTER”
9. After entering Linux, input “sudo update-grub” in the command line to fix grub.
10. Type “sudo grub-install/dev/sda”. //sda is your boot disk.
Method 3. Rebuild MBR with third-party software
If you think it's too complex to type commands one by one to rebuild MBR, you might as well resort to a much more powerful third-party bootable USB creator AOMEI Partition Assistant Standard to create a bootable USB drive on a normal PC to repair OS by rebuilding MBR. Compared with the methods above, it’s much easier with this software and it only takes 3 steps. What’s more, other useful features including USB clone and reinstalling Windows 10 to SSD are also supported.
Free download the most functional bootable USB creator to have a try.
Note: Before you start, you must plug your empty USB drive into a working computer and then, create a bootable USB drive.
Step 1. After successfully booting from the bootable hard drive, you’ll be navigated to the interface of AOMEI Partition Assistant Standard. Right-click on the problematic system disk and select “Rebuild MBR”.
Step 2. Set the type of MBR for the current OS.
Step 3. After returning to the main interface, hit “Apply” on the upper left and then, hit “Proceed” to begin the rebuilding operation.
How to fix “grub rescue unknown file system”? It can be complicated if you go in a wrong direction. In this article, I listed 3 most common ways to help you deal with that issue. Among them, AOMEI Partition Assistant Standard provides the most convenient way. Besides those features mentioned above, if you want to try other more like MBR to GPT OS migration, system disk wiping, and free space allocation, you should upgrade it to pro version.