Here is a quick guide for moving (or merging) the ConfigMgr Content Library from one drive / volume to another using the Content Library Transfer tool (ContentLibraryTransfer.exe). Starting with ConfigMgr 1806, the Content Library Transfer Tool is included with the ConfigMgr installation. Prior to ConfigMgr 1806 you would have to download the ConfigMgr Toolkit to get the tool.
Make sure to run the tool locally on the server (or client) where you have the content library. This can be either the Site Server itself, or one of your distribution points. If you are running this on a DP, make sure that DP is enabled for maintenance mode, so no clients are trying to access it, or so that the site server does not try to distribute content to it.
In this example one of my site servers were running low on disk space on my C drive, and after some research I found that it had stored some of the ConfigMgr Content Library on it. I had simply forgot to add the NO_SMS_ON_DRIVE.SMS file to my OS Volume (C:) when setting up the site server, and when site server was running low on the data volume (E:) it started to use space on my C: drive instead.
Running the tool
Running the tool is easy, simply run it in an elevated command prompt, and specify the source drive, in my case C, and the target drive, in my case E. Here is the full command:
ContentLibraryTransfer.exe -SourceDrive C -TargetDrive E
After the copy of the Content Library content has been completed, the tool will delete any existing Content Library folders on the source drive, and place a NO_SMS_ON_DRIVE.SMS file to prevent ConfigMgr from using it in the future.
Microsoft Docs: Content Library Transfer tool