It is inherently
unsafe to shut down a computer in the middle of what it is
doing, whatever it is. For the most part, this is
due largely to the physical hard
disk and what state the
information transfer is in when the
power goes down. This
message is commonly
associated with the
Windows operating system, even though
Linux and
Macintosh also need to
shut down properly to keep up health. If you are seeing this screen under
Windows, you do not have a "
Power Smart"
motherboard (typically all ATX configurations are), as the computer releases the power through a command to the board. Instead you will need to manually disengage the power.
In a generic sense
Windows and
Linux need to accomplish these tasks in this order to shut down
Many OS's perform a
disk check after
power is
lost, or an
unexpected shutdown occurs, this is to verify disk block integrity. There is no telling where the head of the disk could have been when it performed it's last
action, or what it wrote to, or how far it got before it
died. It's all at the mercy of
entropy, and thus most modern
systems use a disk checking utility if it senses that a disk was not unmounted
cleanly. For
Windows NT there is
chkdsk,
Windows 9x has
scandisk/
scandskw, there is
Macintosh Disk First Aid, and Linux has
fsck. Though this is burdensome and oftentimes yields nothing, it is the
disk errors that pile up that can cause a great amount of lost data and general
instability.
The concept of disk checking on Windows-based
fat16 systems was originally put together by Peter Norton, the
Norton Utilities guy in the form of chkdsk 4.0 for DOS. (Unix had disk checking on their filesystems long before Windows.) Scandisk saw the light of day with DOS 5.0 with the advent of
Smartdisk, disk caching software responsible for many
lost clusters. The
Norton trademark was eventually purchased by Symantec, who created what went on to become
Windows Scandisk). Microsoft, after seeing the benefits of regular
system maintenance,
Scandisk was included with the
software diagnostics that went into the original
Windows 95, even though
Norton kept publishing it's
toolkit (with many added features).
Microsoft licenced this technology from them, no doubt for quite a sum, seeing the distribution of all of the OS's.
It should be noted that shutting down from
single-user mode, or MS-DOS mode (the
VGA graphic you see is on some systems just on top of a
DOS prompt) is usually
fairly safe. There is very little
OS to worry about, and there are not usually a lot of
daemons and behind-the-scenes items to worry about. This is typically the point of these modes.
Windows NT 4.0 brings you to a
VGA GUI screen where it says it is safe to
shut down. It is only holding the bare minimum of drivers in
memory when it does this, and all the disks are unmounted from the
kernel.
Another very
rare, but possible problem with just slamming the
power button off on your
machine while it is running is that of a power
flash across the board. It isn't likely, but it is
possible that the
machine lets go of all the
juice a little funny, and some of it does some damage to the
circuits on the way out. Violently
jerking the power on and off to a
system is not good for it, and can cause serious damage over time. Many computers that have been mistreated you will find have "flukey"
hardware troubles compared to people who maintained their
systems.
Doing what your computer thinks is best is almost never a bad thing when it comes to
maintenance. My
advice to you is to never skip that disk check unless you want to take that chance (or you know it's absolutely wrong). Disk damage on the
file system level can and does happen this way everyday. It's best to stay on top of it before it spreads and causes damage
elsewhere on your system.
Thanks to bane and frater_219 for corrections and clarifications