Microsoft has released an update for Windows 7 pre-beta that resolves a problem that some testers discovered with external NTFS drives. The update improve the reliability and operation of an external NTFS drive according to Microsoft and can be downloaded from Windows Update. A reboot of the computer might be required after installation.
The Microsoft Knowledgebase article is not mentioning the cause of the problem. This is the second update that has been released for a pre-beta build of Windows 7. The first update fixed a security vulnerability that affected all Windows operating systems.