Our hard disks store our programs and, most
importantly, our precious data, so they have a special place in the
computing firmament. They ought to be pampered and coddled to ensure a
long and trouble-free existence, but that’s rarely the case,
unfortunately. Just consider everything that a modern hard disk has to
put up with:
General wear and tear—
If your computer is running right now, its hard disk is spinning away
at between 5,400 and 10,000 revolutions per minute. That’s right, even
though you’re not doing anything, the hard disk is hard at work. Because
of this constant activity, most hard disks simply wear out after a few
years.
The old bump-and-grind— Your hard disk includes read/write heads
that are used to read data from and write data to the disk. These heads
float on a cushion of air just above the spinning hard disk platters. A
bump or jolt of sufficient intensity can send them crashing onto the
surface of the disk, which could easily result in trashed data. If the
heads happen to hit a particularly sensitive area, the entire hard disk
could crash. Notebook computers are particularly prone to this problem.
Power surges— The
current supplied to your PC is, under normal conditions, relatively
constant. It’s possible, however, for massive power surges to assail
your computer (for example, during a lightning storm). These surges can
wreak havoc on a carefully arranged hard disk.
So, what can you do about
it? Windows Vista comes with a program called Check Disk that can check
your hard disk for problems and repair them automatically. It might not
be able to recover a totally trashed hard disk, but it can at least let
you know when a hard disk might be heading for trouble.
Check Disk performs a
battery of tests on a hard disk, including looking for invalid
filenames, invalid file dates and times, bad sectors, and invalid
compression structures. In the hard disk’s file system, Check Disk also
looks for the following errors:
Lost clusters
Invalid clusters
Cross-linked clusters
File system cycles
The next few sections explain these errors in more detail.
Understanding Clusters
Large hard
disks are inherently inefficient. Formatting a disk divides the disk’s
magnetic medium into small storage areas called sectors,
which usually hold up to 512 bytes of data. A large hard disk can
contain tens of millions of sectors, so it would be too inefficient for
Windows Vista to deal with individual sectors. Instead, Windows Vista
groups sectors into clusters, the size of which depends on the file system and the size of the partition, as shown in Table 1.
Table 1. Default Cluster Sizes for Various File Systems and Partition Sizes
| Partition Size | FAT16 Cluster Size | FAT32 Cluster Size | NTFS Cluster Size |
|---|
| 7MB–16MB | 2KB | N/A | 512 bytes |
| 17MB–32MB | 512 bytes | N/A | 512 bytes |
| 33MB–64MB | 1KB | 512 bytes | 512 bytes |
| 65MB–128MB | 2KB | 1KB | 512 bytes |
| 129MB–256MB | 4KB | 2KB | 512 bytes |
| 257MB–512MB | 8KB | 4KB | 512 bytes |
| 513MB–1,024MB | 16KB | 4KB | 1KB |
| 1,025MB–2GB | 32KB | 4KB | 2KB |
| 2GB–4GB | 64KB | 4KB | 4KB |
| 4GB–8GB | N/A | 4KB | 4KB |
| 8GB–16GB | N/A | 8KB | 4KB |
| 16GB–32GB | N/A | 16KB | 4KB |
| 32GB–2TB | N/A | N/A | 4KB |
Still,
each hard disk has many thousands of clusters, so it’s the job of the
file system to keep track of everything. In particular, for each file on
the disk, the file system maintains an entry in a file directory, a sort of table of contents for your files. (On an NTFS partition, this is the Master File Table, or MFT.)
Understanding Lost Clusters
A lost cluster (also sometimes called an orphaned cluster)
is a cluster that, according to the file system, is associated with a
file, but that has no link to any entry in the file directory. Program
crashes, power surges, or power outages are some typical causes of lost
clusters.
If Check Disk comes
across lost clusters, it offers to convert them to files in either the
file’s original folder (if Check Disk can determine the proper folder)
or in a new folder named Folder.000 in the root of the %SystemDrive%. (If that folder already exists, Check Disk creates a new folder named Folder.001 instead.) In that folder, Check Disk converts the lost clusters to files with names like File0000.chk and File0001.chk.
You can look at these
files (using a text editor) to see whether they contain any useful data
and then try to salvage it. Most often, however, these files are
unusable and most people just delete them.
Understanding Invalid Clusters
An invalid cluster is one that falls under one of the following three categories:
A file system
entry with an illegal value. (In the FAT16 file system, for example, an
entry that refers to cluster 1 is illegal because a disk’s cluster
numbers start at 2.)
A file system entry that refers to a cluster number larger than the total number of clusters on the disk.
A file system entry that is marked as unused, but is part of a cluster chain.
In this case, Check
Disk asks whether you want to convert these lost file fragments to
files. If you say yes, Check Disk truncates the file by replacing the
invalid cluster with an EOF (end of file)
marker and then converts the lost file fragments to files. These are
probably the truncated portion of the file, so you can examine them and
try to piece everything back together. More likely, however, you just
have to trash these files.
Understanding Cross-Linked Clusters
A cross-linked cluster
is a cluster assigned to two different files (or twice in the same
file). Check Disk offers to delete the affected files, copy the
cross-linked cluster to each affected file,
or ignore the cross-linked files altogether. In most cases, the safest
bet is to copy the cross-linked cluster to each affected file. That way,
at least one of the affected files should be usable.
Understanding Cycles
In an NTFS partition, a cycle
is a corruption in the file system whereby a subfolder’s parent folder
is listed as the subfolder itself. For example, a folder named C:\Data should have C:\ as its parent; if C:\Data is a cycle, C:\Data—the
same folder—is listed as the parent instead. This creates a kind of
loop in the file system that can cause the cycled folder to “disappear.”
Running the Check Disk GUI
Check Disk has two
versions: a GUI version and a command-line version. See the next section
to learn how to use the command-line version. Here are the steps to
follow to run the GUI version of Check Disk:
1. | In Windows Explorer, right-click the drive you want to check and then click Properties. The drive’s properties sheet appears.
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2. | Display the Tools tab.
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3. | Click the Check Now button and enter your User Account Control credentials. The Check Disk window appears, as shown in Figure 1.
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4. | Activate one or both of the following options, if desired:
Automatically Fix File System Errors—
If you activate this check box, Check Disk automatically repairs any
file system errors that it finds. If you leave this option deactivated,
Check Disk just reports on any errors it finds.
Scan for and Attempt Recovery of Bad Sectors—
If you activate this check box, Check Disk performs a sector-by-sector
surface check of the hard disk surface. If Check Disk finds a bad
sector, it automatically attempts to recover any information stored in
the sector and it marks the sector as defective so that no information
can be stored there in the future.
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5. | |
6. | If
you activated the Automatically Fix File System Errors check box and
are checking a partition that has open system files, Check Disk will
tell you that it can’t continue because it requires exclusive access to
the disk. It will then ask whether you want to schedule the scan to
occur the next time you boot the computer. Click Schedule Disk Check.
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7. | When the scan is complete, Check Disk displays a message letting you know and a report on the errors it found, if any.
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The AUTOCHK Utility
If you click Schedule
Disk Check when Check Disk asks whether you want to schedule the scan
for the next boot, the program adds the AUTOCHK utility to the following Registry setting:
HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\BootExecute
This setting specifies the programs that Windows Vista should run at boot time when the Session Manager is loading. AUTOCHK
is the automatic version of Check Disk that runs at system startup. If
you want the option of skipping the disk check, you need to specify a
timeout value for AUTOCHK. You change the timeout value by adding the AutoChkTimeOut setting as a DWORD value in the same Registry key:
HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\
Set this to the number of seconds that you want to use for the timeout. Another way to set the timeout value is to use the CHKNTFS /T:[time] command, where time is the number of seconds to use for the timeout. (If you exclude time, CHKNTFS returns the current timeout setting.) For example, the following command sets the timeout to 60 seconds:
When AUTOCHK is scheduled with a timeout value greater than 0, you see the following the next time you restart the computer:
A disk check has been scheduled.
To skip disk checking, press any key within 60 second(s).
You can bypass the check by pressing a key before the timeout expires.
Running Check Disk from the Command Line
Here’s the syntax for Check Disk’s command-line version:
CHKDSK [volume [filename]] [/F] [/V] [/R] [/X] [/I] [/C] [/L:[size]]
| | |
|---|
| volume | The drive letter (followed by a colon) or volume name. |
| filename | On FAT16 and FAT32 disks, the name of the file to check. Include the path if the file isn’t in the current folder. |
| /F | Tells
Check Disk to fix errors automatically. This is the same as running the
Check Disk GUI with the Automatically Fix File System Errors option
activated. |
| /V | Runs
Check Disk in verbose mode. On FAT16 and FAT32 drives, Check Disk
displays the path and name of every file on the disk; on NTFS drives,
displays cleanup messages, if any. |
| /R | Tells
Check Disk to scan the disk surface for bad sectors and recover data
from the bad sectors, if possible. This is the same as running the Check
Disk GUI with the Scan for and Attempt Recovery of Bad Sectors option
activated. |
| /X | On
NTFS nonsystem disks that have open files, forces the volume to
dismount, invalidates the open file handles, and then runs the scan (the
/F switch is implied).
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| /I | On NTFS disks, tells Check Disk to check only the file system’s index entries. |
| /C | On
NTFS disks, tells Check Disk to skip the checking of cycles within the
folder structure. This is a rare error, so using /C to skip the cycle
check can speed up the disk check. |
| /L:[size] | On
NTFS disks, tells Check Disk to set the size of its log file to the
specified number of kilobytes. The default size is 65,536, which is big
enough for most systems, so you should never need to change the size.
Note that if you include this switch without the size parameter, Check Disk tells you the current size of the log file. |
