if I fetch the code to a new fetch destination dir, the build will be successful.
if I fetch the code to the old fetch destination dir(contain the code fetching last time and uncheck the option "Delete directory before loading" in build definition), the build will fail.

paste the error log
================================================
2009-05-14 23:16:19 Invoking pre-build participant "com.ibm.team.build.jazzscm"
Accepting changes into workspace "dev Workspace" ...
Fetching files to fetch destination "D:\dev\Build" ...
org.eclipse.core.runtime.AssertionFailedException: assertion failed:
at org.eclipse.core.runtime.Assert.isTrue(Assert.java:109)
at org.eclipse.core.runtime.Assert.isTrue(Assert.java:95)
at com.ibm.team.repository.common.internal.content.util.BTreeHeap.removeFromFree(BTreeHeap.java:243)
at com.ibm.team.repository.common.internal.content.util.BTreeHeap.doFree(BTreeHeap.java:323)
at com.ibm.team.repository.common.internal.content.util.BTreeHeap.free(BTreeHeap.java:283)
at com.ibm.team.filesystem.client.internal.PersistentHeapManager$AutoClosingPersistentFileHeap.free(PersistentHeapManager.java:128)
at com.ibm.team.repository.common.internal.content.util.DiskBackedHashMap.freeObject(DiskBackedHashMap.java:406)
at com.ibm.team.repository.common.internal.content.util.DiskBackedHashMap$Entry.setValue(DiskBackedHashMap.java:965)
at com.ibm.team.repository.common.internal.content.util.CachedDiskBackedHashMap$CachedEntry.flush(CachedDiskBackedHashMap.java:353)
at com.ibm.team.repository.common.internal.content.util.CachedDiskBackedHashMap.flushCache(CachedDiskBackedHashMap.java:160)
at com.ibm.team.repository.common.internal.content.util.PersistentDiskBackedHashMap.persist(PersistentDiskBackedHashMap.java:145)
at com.ibm.team.repository.common.internal.content.util.PersistentDiskBackedHashMap.close(PersistentDiskBackedHashMap.java:139)
at com.ibm.team.filesystem.client.internal.Store.close(Store.java:54)
at com.ibm.team.filesystem.client.internal.LockableMap.close(LockableMap.java:151)
at com.ibm.team.filesystem.client.internal.SharingMetadata2$MetadataDiskBackedMapManager.closeAll(SharingMetadata2.java:748)
at com.ibm.team.filesystem.client.internal.SharingMetadata2.close(SharingMetadata2.java:1379)
at com.ibm.team.filesystem.client.internal.MetadataChangeTracker.close(MetadataChangeTracker.java:349)
at com.ibm.team.filesystem.client.internal.copyfileareas.CopyFileAreaStore.sync(CopyFileAreaStore.java:2693)
at com.ibm.team.filesystem.client.internal.operations.LoadOperation.doLoad(LoadOperation.java:437)
at com.ibm.team.filesystem.client.internal.operations.LocalFileSystemLoadOperation.execute(LocalFileSystemLoadOperation.java:97)
at com.ibm.team.filesystem.client.internal.operations.FileSystemOperation.run(FileSystemOperation.java:89)
at com.ibm.team.build.internal.scm.SourceControlUtility.updateFileCopyArea(SourceControlUtility.java:279)
at com.ibm.team.build.internal.engine.JazzScmPreBuildParticipant.preBuild(JazzScmPreBuildParticipant.java:152)
at com.ibm.team.build.internal.engine.BuildLoop.invokePreBuildParticipants(BuildLoop.java:570)
at com.ibm.team.build.internal.engine.BuildLoop$2.run(BuildLoop.java:426)
at java.lang.Thread.run(Thread.java:735)