Lorne, here is what I get:

MacBook-Pro-4:~ Leo_Lazo$ sudo gpt -vv -r show /dev/disk0

Password:

gpt show: /dev/disk0: mediasize=500277790720; sectorsize=512; blocks=977105060

gpt show: /dev/disk0: PMBR at sector 0

gpt show: /dev/disk0: Pri GPT at sector 1

gpt show: /dev/disk0: Sec GPT at sector 977105059

      start       size  index  contents

          0          1         PMBR

          1          1         Pri GPT header

          2         32         Pri GPT table

         34          6        

         40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B

     409640  975425848      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC

  975835488    1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC

  977105024          3        

  977105027         32         Sec GPT table

  977105059          1         Sec GPT header

Then

MacBook-Pro-4:~ Leo_Lazo$ sudo gdisk /dev/disk0

Password:

GPT fdisk (gdisk) version 1.0.1

Warning: Devices opened with shared lock will not have their

partition table automatically reloaded!

NOTE: Write test failed with error number 1. It will be impossible to save

changes to this disk's partition table!

You may need to deactivate System Integrity Protection to use this program. See

https://www.quora.com/How-do-I-turn-off-the-rootless-in-OS-X-El-Capitan-10-11

for more information.

Partition table scan:

  MBR: protective

  BSD: not present

  APM: not present

  GPT: present

Found valid GPT with protective MBR; using GPT.

Command (? for help)

Not sure what to do next. Thanks for your help

I picked disk0 as an example. You need to use the /dev/diskN where N points to your 1TB disk.

I got this after trying to run this:

Command (? for help): sudo gdisk /dev/disk2

You may need to edit /etc/fstab and/or your boot loader configuration!

You cannot type Gdisk commands inside Gdisk commands. Press q and go back to Terminal command. Once at the Terminal prompt, run diskutil list and find out which disk is your 1TB disk. Once you know, run

sudo gdisk /dev/diskN (where N is pointing to the 1TB disk).

Did it and here is what I get:

GPT fdisk (gdisk) version 1.0.1

Problem opening /dev/disk2 for reading! Error is 16.

1. Does diskutil list show the disk in the output?

2. Can you mount the disk and test?

I unmounted the disk and mounted it again then I ran:

MacBook-Pro-4:~ Leo_Lazo$ sudo gdisk /dev/disk2

Password:

GPT fdisk (gdisk) version 1.0.1

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: not present

  BSD: not present

  APM: not present

  GPT: damaged

Found invalid MBR and corrupt GPT. What do you want to do? (Using the

GPT MAY permit recovery of GPT data.)

1 - Use current GPT

2 - Create blank GPT

Your answer:

So which one do I use?

Use option 2 - Create a Blank GPT. Once Gdisk creates a new GPT, use 'p'(rint),  'w'(rite), 'y(es - to confirm the write). Please post screen the output after you execute the commands.

We are going to erase the disk.

Command (? for help): p

Disk /dev/disk2: 1953525168 sectors, 931.5 GiB

Logical sector size: 512 bytes

Disk identifier (GUID): FD5B4614-D6C5-4634-AE82-6BB7B59B95D1

Partition table holds up to 128 entries

First usable sector is 34, last usable sector is 1953525134

Partitions will be aligned on 2048-sector boundaries

Total free space is 1953525101 sectors (931.5 GiB)

Number  Start (sector)    End (sector)  Size       Code  Name

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING

PARTITIONS!!

Do you want to proceed? (Y/N): Y

OK; writing new GUID partition table (GPT) to /dev/disk2.

Warning: The kernel may continue to use old or deleted partitions.

You should reboot or remove the drive.

The operation has completed successfully.

MacBook-Pro-4:~ Leo_Lazo$

Hi Loner,

After the commands, I did not see the disk mounted. I use the command to mount disk and it said successful but still didn't see. I then rebooted the MacBook Pro and still no disk. So I used Disk Utilities to erase disk again and not all is good.

Thanks so much for all your help. I do have one more question, which is what got me in the first place. Is there a way to actually install Bootcamp & any windows from an external drive and then run windows externally. I do not have a need to use Windows all the time. Honestly only for a few games I like to play every once in a while. That is why I don't want to repartition my internal HD for this.

Thanks again for your help.

You said 'not all good', but I presume you meant 'now all good'.

My recommendation is to create a minimal (40-60GB) Windows installation on the internal disk, and use an external disk as D: (or X: - take your pick) and install all your applications on the external disk. You do not run into licensing issues with Windows being on an external disk. D: could be a 1TB NTFS drive. As long as the external disk is connected with a USB3/TB/FW800 connection, it should perform reasonably well.

Thanks for your advice AND for your walkthrough in reformatting my drive. If you're ever on this neck of the woods (Shanghai, China) dinner is on me!

Thanks for your kind offer. . If you run into any other/further issues, please post back. The community will do their utmost to assist.

Hey gcarey3 thank you very much, that single line solve my problem, my USB is now alive You rock! Cheers.

I registered just to say thank you.

You're a lifesafer.