Converting back from GBT to MBR without having access to an OS, and without losing data

Question

I was trying to install Slackware linux from bootable USB stick on my machine alongside windows 8. My partition table was MBR, So it does not support more than 4 primary partitions. Unfortunately I already had 3 and I need other two partitions one for linux system and the other for linux swap.

So i converted to GBT with: gfdisk /dev/sda

Which i think that was a mistake. Anyway, the setup was completed successfully then i restarted my laptop to find there is no boot page to windows neither slackware (Missing operating system) message. I tried after that to convert back MBR using bootable partition magic USB but it can't convert from GBT to MBR. I also tried to fix MBR from windows 8 cd but also didn't work.

Is there any way to convert back to mbr without losing data?

Answer 1

First let me add some in formation for future setups:

1. MBR can support more then four partitions. Just not four primary partitions. Instead you can use the empty MBR record to set up an extended partions and create logical partitions in that.
2. I am pretty sure I have setup slackware in the past using logical partitions. (even for root and swap).

Thus if you manage to recover your windows installation than you can still trivially add a slackware installation.

3. Before converting to GPT alway make sure your have a system capable of booting from that. Usually this requires:

   • an EFI firmware, or
   • an advanced BIOS firmware which understands GPT
   • And an EFI system partition (which is what actually gets booted)

With that background out of the way, lets continue to the rescue part.

If you do not have up to date backups then stop right now. You want to have backups before doing any more write actions to the disk. Which means making a copy of the disk. This is usually done by taking out the disk, adding it to another computer or external USB to SATA case and using something like dd. If you have a second system on the network you can also boot from a live CD and combine dd with gzip and netcat. How to do that is well described in several posts here on [SU].

If you do have a backup then just restore that. Safest, easiest way.

If you do have a backup and like to skip as much work as possible, with the risk of leaving something strange whioch might cause problems in the future: try some rescuing software while booted from a live CD or live USB.

(Leaving specific product recommendations out of the post since a search on recover mbr partition table yields plenty of answers.)