When you attempt to upgrade a computer with a mounted compressed drive to Windows Millennium Edition, one of the following error messages is displayed:

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Drive C, Your Windows Drive, or the Drive You Are Installing to Is Compressed

Setup
Windows is installed on a compressed drive and cannot be upgraded to Windows Millennium Edition until the compression is removed. Please use DriveSpace to remove the compression on the drive.
Message SU0320

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Another Drive Is Compressed

The drive 'X:\' is compressed. If you continue installing Windows Millennium, you will not be able to access the files on the compressed drive after installation is complete. To make the information on this drive available in Windows Millennium, remove the compression on the drive, and then install Windows Millennium.

To continue installing Windows Millennium even though the files on drive 'X:\' will not be available after installation, click Yes.

To stop installing Windows Millennium and return your computer to its original state click No.

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This issue can occur because due to architectural changes in Windows Millennium Edition (such as the removal of real mode support), if you have compressed drive C or your Windows drive you will not be able to install Windows Millennium Edition. If you have other compressed drives on your computer you will be warned that they will not be accessable after upgrading to Windows Millennium Edition.

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To resolve this issue, use the DriveSpace tool to remove the compression on the drive and then run Setup again:

1.	Click Start, point to Programs, point to Accessories, point to System Tools, and then click DriveSpace.
2.	Click to select the compressed drive.
3.	On the Drive menu, click Uncompress, and then follow the instructions on the screen. NOTE: You may want to back up any important data before you uncompress the DriveSpace volume.

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This behavior is by design.

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The DriveSpace utility in Windows Millennium Edition supports mounting (for read/write access) existing compressed removable drives only and cannot be used to compress, mount, or uncompress hard disk drives or compress non-compressed removable drives.

You will not receive a warning for any compressed hard drives that are not mounted when you run Windows Millennium Edition Setup. To access unmounted compressed drives after Windows Millennium Edition is installed, you can use the Drvspace /mount command at an MS-DOS Prompt or start your computer with a Windows 95 or Windows 98 Startup Disk and use ScanDisk to mount the compressed drive in real mode.

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