I have just had the same problem with my MacBook Pro with a 2TB WD Hard Drive with 4 separate partitions which all became "custom" locked a few days ago. Major problem as this had all my Pro Tools and Sound Librarys on this disk
I found a solution from a user called IEatApples in a string which had been archived in this forum. The solution has worked for me, and all my partitions are now unlocked. Here is the URL to the archived forum on the subject
https://forums.macrumors.com/archive/index.php/t-417647.html
I am extremely grateful to IEatApples and indeed this entire archived string. My drive and all partitions are now all unlocked.
Here is IEatApples solution and instructions repeated below which worked for me.
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I called Apple and they gave me the solution

:
http://discussions.apple.com/thread.jspa?messageID=9035132�
I found a solution that worked.
In Terminal type the following sequence (obviously replace "Volumename with the name of your volume):
sudo chflags 0 /Volumes/Volumename
sudo chown 0:80 /Volumes/Volumename
sudo chmod 775 /Volumes/Volumename
sudo chmod -N /Volumes/Volumename
but if your hard drives name includes a [space] like this: "My HD", then you must write the command like this:
sudo chflags 0 /Volumes/firstname\ secondname
sudo chown 0:80 /Volumes/firstname\ secondname
sudo chmod 775 /Volumes/firstname\ secondname
sudo chmod -N /Volumes/firstname\ secondname
or like in my example:
sudo chflags 0 /Volumes/My\ HD
sudo chown 0:80 /Volumes/My\ HD
sudo chmod 775 /Volumes/My\ HD
sudo chmod -N /Volumes/My\ HD
It saved both my drives! THANK YOU APPLE!!!
Thank You IEatApples - you saved me substantial grief
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I repeat, original archived string on this subject in this forum can be found at
https://forums.macrumors.com/archive/index.php/t-417647.html
//FYOS