Our hard disks store our programs and, most
important, 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 probably 7,200 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.
Note
I should say that your hard disk is probably
spinning away as I speak. Windows 7 is actually configured out of the
box to put your hard disk to sleep after 20 minutes of inactivity, so
your hard disk may be resting. If you want to change the hard disk
sleep interval (personally, I turn it off on my machines to improve
performance), select Start, type power,
and then click Change Power-Saving Options in the search results.
Select the power plan you want to use, click Change Plan Settings, and
then click Change Advanced Power Settings, Open the Hard Disk branch,
open the Turn Off Hard Disk After branch, and then set the interval you
prefer.
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 7 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 7 to deal with individual sectors. Instead, Windows 7 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 |
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| 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 |
| 1025MB–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 such as 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. | Select Start, Computer, right-click the drive you want to check, and then click Properties. The drive’s property sheet appears.
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2. | Display the Tools tab.
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3. | Click the Check Now button. 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. | Click Start.
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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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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 7 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.
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