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’t erase it), and Archive. These attributes were first used in FAT-formatted drives in the DOS era, but they are still completely supported by all file formats. The archive bit works basically like this: Whenever a file is saved, the archive bit is turned on. Simply opening a file affects the current state of the archive bit. Backup programs usually turn off a file’s archive bit when the file is backed up. In theory, if a file’s archive bit is turned off, there’s a good backup of that file on some tape. If the archive bit is turned on, it means that the file has been changed since it was last backed up (see Figure 17-44).

Figure 17-44 The archive bit on these files is on

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NOTE Windows Explorer (My Computer in Windows XP, Computer in Vista) by default does not show much about files in any view, even when you select Details from the View menu. The Details view is highly customizable, however, and can reveal a phenomenal amount and variety of information about files.

To customize your view, right-click the column bar (the gray bar that says Name, Size, Type, Date Modified, and so forth) to look at the default choices. You’ll see everything from Attributes, Owner, Author, and Title to file-type specific information such as Genre, Duration, and Bit Rate (for music files). If the default extra view options don’t get your motor revving, selecting the More option brings up a menu offering many more view options! For the purposes of this section, click the Attribute box to display file and folder attributes.

Archive bits are used to perform backups that are not full backups. The following backup types are most often supported:

A normal backup is a full backup. Every file selected is backed up, and the archive bit is turned off for every file backed up. This is the standard “back it all up” option.

A copy backup is identical to a normal backup, with the important distinction being that the archive bits are not changed. This is used (although not often) for making extra copies of a previously completed backup.

An incremental backup includes only files with the archive bit turned on. In other words, it copies only the files that have been changed since the last backup. This backup turns off the archive bits.

A differential backup is identical to an incremental backup, except that it doesn’t turn off the archive bits.

A daily backup, also known as a daily copy backup, makes copies of all the files that have been changed that day. It does not change the archive bits.

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EXAM TIP Be sure you know the types of backups, including which ones change the archive bits and which ones do not.

The motivation for having both the incremental and differential backups may not be clear at first glance—they seem so similar as to be basically the same. Incremental seems the better option at first. If a file is backed up, you would want to turn off the archive bit, right? Well, maybe. But there is one scenario where that might not be too attractive. Most backups do a big weekly normal backup, followed by daily incremental or differential backups at the end of every business day. Figure 17-45 shows the difference between incremental and differential backups.

Figure 17-45 Incremental versus differential

Notice that a differential backup is a cumulative backup. Because the archive bits are not set, it keeps backing up all changes since the last normal backup. This means the backup files will get progressively larger throughout the week (assuming a standard weekly normal backup). The incremental backup, by contrast, only backs up files changed since the last backup. Each incremental backup file will be relatively small and also totally different from the previous backup file.

Let’s assume that the system is wiped out on a Thursday morning. How can you restore the system to a useful state?

If you’re using an incremental backup, you will first have to restore the last weekly backup you ran on Monday, then the Tuesday backup, and then the Wednesday backup before the system is restored to its Thursday morning state.