NOTE: This only works on NTFS file systems, FAT* does not support it.
DLP uses absolute paths, so if you started a project on the C: drive, and then run out of space, you cannot simply move it to the D: drive and continue working: When you open the project in DLP, it will complain about each file in the project not being found on the C: drive. Using DLP's export function can work, but not if you have many photo's with the same names in directories - all the files get moved to one directory.
Using NTFSlink to create a symbolic link is the answer. Symbolic links are similar to shortcuts, but shortcuts doesn't work here. [NTFSlink]
If your project was created in C:\project1, and you move it to D:\newproject, right-click the newproject folder in D:\ and drag it to C:\, and select "Create hardlink here" (it will actually create a softlink, but for us that is immaterial). Then rename the created link to project1. After this, to DLP it will look as if the newproject directory is actually the project1 directory on C:\