Uninitialized HDD after GPT to MBR conversation, 3 HFS+ Volumes and 1 ExFAT 2 TB External Hard Drive. 

Error during converting from GPT to MBR and now Disk can't be mounted. 

'The disk you inserted was not readable by this computer' appears when HDD is plugging in. Files are really important for me.

I read a lot of questions and answers here, but still can't find a solution that can help. Here is what I got in the terminal using commands from some answers near. It seems like partition table is broken. Is that possible to recover or rebuild it? If there is already clear answer somewhere here, coud someone push me there or maybe give me a vector, where I should search in? If someone could help for this specific issue, it will be really awesome. I will be really appreciate any help.

    sudo fdisk /dev/disk6
    Disk: /dev/disk6    geometry: 243201/255/63 [3907029168 sectors]
    Signature: 0xAA55
             Starting       Ending
     #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
    ------------------------------------------------------------------------
     1: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
     2: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
     3: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
     4: 00    0   0   0 -    0   0   0 [         0 -          0] unused  


    gpt show disk6
           start        size  index  contents
               0           1         MBR
               1           1         Pri GPT header
               2          32         Pri GPT table
              34        2014         
            2048  1562503168      1  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
      1562505216  1326149632      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
      2888654848   236972032      3  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
      3125626880   781402112      4  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
      3907028992         143         
      3907029135          32         Sec GPT table
      3907029167           1         Sec GPT header


       diskutil info disk6
       Device Identifier:         disk6
       Device Node:               /dev/disk6
       Whole:                     Yes
       Part of Whole:             disk6
       Device / Media Name:       ST2000LM007-1R8174

       Volume Name:               Not applicable (no file system)
       Mounted:                   Not applicable (no file system)
       File System:               None

       Content (IOContent):       None
       OS Can Be Installed:       No
       Media Type:                Generic
       Protocol:                  SATA
       SMART Status:              Verified

       Disk Size:                 2.0 TB (2000398934016 Bytes) (exactly 3907029168 512-Byte-Units)
       Device Block Size:         512 Bytes

       Read-Only Media:           No
       Read-Only Volume:          Not applicable (no file system)

       Device Location:           External
       Removable Media:           Fixed

       Solid State:               No
       Virtual:                   No
       Hardware AES Support:      No


Please, let me know if there will be a need in more information, I'll do my best.
Thanks for your attention.
Having rummaged on other forums and sites, I came to the conclusion that gdisk can save me. And lo and behold! 

Here are my actions, maybe it will save someone:

sudo gdisk /dev/disk6
GPT fdisk (gdisk) version 1.0.4

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

Found valid MBR and GPT. Which do you want to use?
 1 - MBR
 2 - GPT
 3 - Create blank GPT

Your answer: 2
Using GPT and creating fresh protective MBR.

Command (? for help): p
Disk /dev/disk3: 3907029168 sectors, 1.8 TiB
Sector size (logical): 512 bytes
Disk identifier (GUID): 366A9BED-8C95-4B28-07DC-45407D80EFA7
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 3907029134
Partitions will be aligned on 2048-sector boundaries
Total free space is 2157 sectors (1.1 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048      1562505215   745.1 GiB   AF00  
   2      1562505216      2888654847   632.4 GiB   AF00  
   3      2888654848      3125626879   113.0 GiB   AF00  
   4      3125626880      3907028991   372.6 GiB   0700  Basic data partition

Command (? for help): ?
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

Command (? for help): r

Recovery/transformation command (? for help): ?
b   use backup GPT header (rebuilding main)
c   load backup partition table from disk (rebuilding main)
d   use main GPT header (rebuilding backup)
e   load main partition table from disk (rebuilding backup)
f   load MBR and build fresh GPT from it
g   convert GPT into MBR and exit
h   make hybrid MBR
i   show detailed information on a partition
l   load partition data from a backup file
m   return to main menu
o   print protective MBR data
p   print the partition table
q   quit without saving changes
t   transform BSD disklabel partition
v   verify disk
w   write table to disk and exit
x   extra functionality (experts only)
?   print this menu

Recovery/transformation command (? for help): g

MBR command (? for help): ?
a   toggle the active/boot flag
c   recompute all CHS values
l   set partition as logical
o   omit partition
p   print the MBR partition table
q   quit without saving changes
r   set partition as primary
s   sort MBR partitions
t   change partition type code
w   write the MBR partition table to disk and exit

MBR command (? for help): p

** NOTE: Partition numbers do NOT indicate final primary/logical status,
** unlike in most MBR partitioning tools!

** Extended partitions are not displayed, but will be generated as required.

Disk size is 3907029168 sectors (1.8 TiB)
MBR disk identifier: 0x00000000
MBR partitions:

                                                   Can Be   Can Be
Number  Boot  Start Sector   End Sector   Status   Logical  Primary   Code
   1                  2048   1562505215   primary     Y        Y      0xAF
   2            1562505216   2888654847   primary              Y      0xAF
   3            2888654848   3125626879   primary              Y      0xAF
   4            3125626880   3907028991   primary              Y      0x07


MBR command (? for help): w

Converted 4 partitions. Finalize and exit? (Y/N): y
Warning: The kernel may continue to use old or deleted partitions.
You should reboot or remove the drive.
GPT data structures destroyed! You may now partition the disk using fdisk or
other utilities.


That's it, Drive is fully restored. Insane.