I just got a Windows startup error on boot that says "BOOTMGR is compressed." What happened?
BOOTMGR, or Boot Manager, is a hidden system file that's part of the Windows startup process. At startup, the boot sector on your system drive loads BOOTMGR, which then looks for an active partition with a BCD (Boot Configuration Data) file. If the Windows Boot Manager can't be read for some reason, the boot process is interrupted.
The most common reason for a problem with BOOTMGR is that it's damaged or missing. Sometimes, though, BOOTMGR can be compressed and left in an unreadable state as well.
One of the long-standing features of NTFS volumes is in-place file compression, which allows sparse files or easily compressible data (such as plaintext) to be stored efficiently. Over time, though, storage has become so cheap that the need for on-disk compression has waned, and third-party programs can provide far better compression ratios than something integrated with the file system.
That said, the on-disk compression system in NTFS remains available. Unfortunately, if it's used mistakenly to compress the BOOTMGR file -- which is, after all, just another NTFS file -- then it renders the Windows Boot Manager useless. Badly written third-party "system optimization" tools are notorious for doing things like this.
The best way to fix this in Windows Vista and up is to boot into the Recovery Console and submit the following commands in this order:
The last command will prompt you to add the current Windows installation to the boot list; say, "Yes" to this option, and reboot.
If this fails, boot to the Recovery Console again and issue the command compact /u /a c:*.* This will attempt to undo all of the file-system-level compression on the C: drive (assuming C: is your system drive), and it may take quite some time to complete.
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This was first published in May 2013