Content Distribution model is significantly changed in SCCM 2012. Well-known SCCM 2007 procedures for moving DP content from one disk to another does not work for 2012 as a result.
Fortunately Microsoft published ContentLibraryTransfer.exe tool (a part of SCCM ConfigMgr 2012 SP1 Toolkit available for download here). This tool can be used for the content transfer.
Note: In the toolkit description Microsoft listed SCCM 2012 SP1 as a supported product; so I assume it may not work or at least not supported for SCCM 2012 RTM.
ContentLibraryTransfer –SourceDrive <Letter> –TargetDrive <Letter>
1. The tool must run locally on the distribution point; it cannot be run from a remote machine.
2. The tool must run only when the distribution point is not actively being accessed by client computers. If the tool is run while client computers are accessing the content, the content library on the destination drive may have incomplete data or the data transfer might fail altogether leading to an unusable content library.
3. The tool must only run when no content is being distributed to the distribution point. If the tool is run while content is being written to the distribution point, the content library on the destination drive may have incomplete data or the data transfer might fail altogether leading to an unusable content library.
You can read detailed instructions in documentation for the Toolkit.