changing team.scm.fetchDestination caused build failure

First I setup the load directory at E:\CAC_71_BUILD, the build succeeded. Then changed the load directory to E:\CAC_71_BUILD\CAC_SRC, got the following errors:

com.ibm.team.filesystem.client.FileSystemStatusException: Status ERROR: com.ibm.team.filesystem.client code=0 Cannot create sandbox at E:/CAC_71_BUILD/CAC_SRC because one already exists at E:/CAC_71_BUILD null
    at com.ibm.team.filesystem.client.internal.copyfileareas.CopyFileAreaManager.createCopyFileArea(CopyFileAreaManager.java:309)
    at com.ibm.team.filesystem.client.internal.copyfileareas.CopyFileAreaManager.createCopyFileArea(CopyFileAreaManager.java:1)
    at com.ibm.team.filesystem.client.internal.SharingManager.register(SharingManager.java:1218)
    at com.ibm.team.build.internal.engine.JazzScmPreBuildParticipant.preBuild(JazzScmPreBuildParticipant.java:188)
    at com.ibm.team.build.internal.engine.BuildLoop.invokePreBuildParticipants(BuildLoop.java:881)
    at com.ibm.team.build.internal.engine.BuildLoop$2.run(BuildLoop.java:685)
    at java.lang.Thread.run(Thread.java:736)

Not sure why this happened.