One frustrating Windows 2000 (Win2K) limitation is that the OS doesn't support earlier versions of NTFS. You must convert an NTFS volume to Win2K's NTFS 5.0 (NTFS5) format so that Win2K can talk to an NTFS volume. However, this process isn't manual—Win2K automatically converts an NTFS volume that an earlier Windows NT version created to the NTFS5 format the moment Win2K mounts the volume. Also, Win2K performs this conversion without first prompting you for confirmation. The good news is that the NTFS5 conversion process is fast (even on large NTFS volumes) and doesn't require you to reboot the system after the conversion is complete. The bad news is that this automatic conversion can cause you serious headaches if you aren't working in a pure Win2K environment. However, the automatic-conversion behavior affects only local drives that are connected to the system in question and doesn't apply to network-connected NTFS 4.0 (NTFS4) volumes that Win2K won't modify or convert.

If you're running a multiboot system that also houses NT 4.0 or you shuttle NTFS-formatted removable media back and forth between Win2K and NT 4.0, you need to know certain facts about NTFS5. For NT 4.0 to work with an NTFS5 volume, you must install Service Pack 4 (SP4) or later on all NT 4.0 installations before you install Win2K. NT 4.0 SP3 and earlier systems won't work with these volumes and will crash at startup if Win2K converts their NTFS4 boot partitions to NTFS5. Also, the NTFS4 to NTFS5 format conversion is a one-way operation. You can't convert volumes back to NTFS4, even if you use third-party utilities. (In fact, the only way you can remedy this situation is to make a system backup and then restore the data after you rebuild your partitions from scratch and reinstall NT 4.0.) After you convert your NTFS4 volumes to the NTFS5 format, you'll discover that you can't install (or reinstall) NT 4.0 on your system and that the setup process will fail. Your system is essentially frozen with respect to its NT 4.0 support and can use only the SP4 or later installations. Finally, because NT installations (even those running SP4 or later) can't run Chkdsk against NTFS5 volumes, you must boot to Win2K to repair disks. A notification to this effect appears when you boot an NT 4.0 installation on a system that contains an NTFS5 volume with its dirty flag bit set, which tells you that you need to run Chkdsk.

Many users will want to install Win2K side by side with their production NT 4.0 systems to evaluate and test the new OS. However, installing Win2K in this manner can affect your NT 4.0 installation. Because you can't reinstall NT 4.0 on the system or use Chkdsk to manage the newly converted NTFS5 volumes from NT 4.0, you severely limit your ability to maintain your NT 4.0 installations. I've found that after you perform the one-way conversion, your NT 4.0 installation gradually becomes more unstable and you can't reinstall NT 4.0. (I learned this lesson the hard way after I installed Win2K on a separate volume on my primary NT 4.0 workstation.) If you won't be using NT 4.0 much longer, you might not be concerned about the Win2K installation consequences. However, if you don't plan to make a final conversion to Win2K for some time and you want to maintain your production NT 4.0 installations, you might want to think twice about installing Win2K on your production systems. Instead, install Win2K on only dedicated evaluation machines or wait until you're ready to make a wholesale migration.