MacOS – How to remove OS X Installer for 10.10. to get back to 10.7, the originally installed OS

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I ran OS X 10.7 latest version on my MacBookPro 2011.
Then I installed on an external USB stick 10.10.3 from my running system OS X 10.7.
The installation on the stick was successful.

After testing 10.10 for some minutes I restarted the system, took of the usb-stick and wanted to restart from my internal disk the 10.7 system.
However, now there is the installer and insists on installing 10.10, what I do not want to do.

The disk utility says that my internal disk is not writable and that I cannot modify it.
Starting with an external disk with 10.7 works, but does not allow to reactivate the internal disk.

Recovery with Cmd ⌘ R loads 10.7 via the internet (it therefore still know that the original system is 10.7), but I cannot recover the 10.7 system, since I cannot write to the internal disk.

The internal disk is now Master Boot Record partitioned.
I do not know if it was GUID before or not: At least it worked for three years before.
I have a backup, but since this is 1TB I do not want to load it via a USB 2.0 port.

How can I reactivate my internal disk with 10.7?
If reasonable I would change to 10.8 (but not higher) if there is a utility that works with 10.8 only.
Any proposal or links to tool to remove the OS X installer and allow me to restart from my internal disk with 10.7?

The output of gpt -r show /dev/disk0:

sudo gpt -r show /dev/disk0 
results in:
       start        size  index  contents
           0           1         MBR
           1           1         
           2  1466246608      1  MBR part 175
  1466246610           1         
  1466246611   487278557      2  MBR part 175

I have a time machine backup.

Best Answer

Something really, really bad happened to your internal 1 TB drive:

The GUID partition table was somehow replaced by a dual (and therefore additionally false) MBR partition table.

You may recover the previous GUID and your missing volume with a disk recovery tool like Disk Warrior to some extent. But since you have a backup just restore it:

  • In case you have a Time Machine backup:

    • Boot to Internet Recovery Mode by pressing altcmdR. The prerequisites for an Internet Recovery are listed here: Recovery/Internet Recovery. In a few words you need the latest firmware update installed, either ethernet or WLAN (WPA/WPA2) and a router with DHCP activated for a successful internet recovery.
      On a 50 Mbps-line it takes about 4 min (presenting a small animated globe) to boot into the Recovery Netboot image which is loaded from an Apple server.

      (Alternatively you may boot to an external drive containing a Recovery HD or a full OS X system or a OS X thumb drive.)

    • Open "Disk Utility" and partition your internal disk: GUID partition scheme (use the "Options..." button at the bottom of the main pane for this), one partition, journaled HFS+.
    • Close "Disk Utility".
    • Attach your Time Machine backup drive.
    • Open "Restore from Time Machine Backup" and restore your main drive if you have booted from a Recovery drive.
      If you've booted from a full OS X external drive just open Time Machine and restore your internal drive.

  • In case you have a different back-up app:

    • Follow the instructions of the non-Apple back-up application
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