At boot-time the storage on the NetManager is checked. If the machine was shutdown properly, then the filesystems will be recognised as clean and so will not be checked. If, however, the computer was just switched off, all local storage will be checked in detail (a 5-stage process). If you have large amounts of storage, this may take a considerable time during which the computer will appear to be doing nothing (though its hard drive light will be on pretty much solidly). If you type Ctrl-T you will get a status report and percentage complete if you want to reassure yourself that it has not locked up!
Occasionally, the filesystem check will reveal problems that the automated mechanism will not fix. If this happens, you will see a message
Automatic file system check failed and the NetManager will enter Single-User Mode (characterised by the message
Enter pathname of shell or RETURN for /bin/sh:).
Hit RETURN to get to a # single-user command prompt. If you are asked for a terminal type, just hit RETURN again. If the suggested terminal type is displayed as
[unknown], enter vt100 and hit RETURN. At the single-user command prompt, type:
This will rescan all filesystem and attempt to fix any problems. When the process is complete, type
reboot to re-attempt a normal boot.
Forcing a filesystem check
The procedure is documented here.