Gdisk error message: Caution: invalid main GPT header, but valid backup; regenerating main header from backup!

At some point when experimenting with partitions on your disk you may get the following error message.

% gdisk /dev/sda
GPT fdisk (gdisk) version 0.7.1

Caution: invalid main GPT header, but valid backup; regenerating main header
from backup!

Caution! After loading partitions, the CRC doesn't check out!
Warning! Main partition table CRC mismatch! Loaded backup partition table
instead of main partition table!

Warning! One or more CRCs don't match. You should repair the disk!

Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: damaged

Found valid MBR and corrupt GPT. Which do you want to use? (Using the
GPT MAY permit recovery of GPT data.)
 1 - MBR
 2 - GPT
 3 - Create blank GPT

As always you have to be very careful depending what you did and what do you want to do as next.

The three options for gdisk gives you the choice what partition tables you will examine. Fortunately for us gpart is not going to write on disk anything until we say so by executing the w (write) command (example bellow). The three options only influences how the data on the disk are going to be interpreted and presented for our review.

# gdisk /dev/sda
Command (? for help): h
b    back up GPT data to a file
c    change a partition's name
d    delete a partition
i    show detailed information on a partition
l    list known partition types
n    add a new partition
o    create a new empty GUID partition table (GPT)
p    print the partition table
q    quit without saving changes
r    recovery and transformation options (experts only)
s    sort partitions
t    change a partition's type code
v    verify disk
w    write table to disk and exit
x    extra functionality (experts only)
?    print this menu

At this stage it is save to experiment and have a look how the partition tables look like. Depending on your choise different partitions may be printed ('p') .

REMEMBER to always quit the session with the 'q' (quit) and never with 'w' otherwise your experiments will be permanently saved on the disk.

In my case this error was misleading and confusing only. I could verify that my current partition schema was not GPT at all but the good old MSDOS one [1]. At this stage I knew that 'gdisk' is not the tool I wanted to use and I finished creating new partitions with 'parted'. I didn't have to use gdisk at all.

The reason why I had some parts of GPT data on my disk is unknown. I can only suspect that it was created when I played with the Windows tool 'EasyBCD'.

I could try to delete the GPT data but base on [2] never had to do it.

References
[1] How to find what type of disk partition schema do I use (msdos, gpt)

[2] Wiping Out Old GPT Data