Apple PowerBook (Duo 270C) - Manual (Page 130)

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If the hard disk is external, make sure it is turned on and its
cable is connected firmly; then restart the PowerBook.
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Check the ID numbers of all SCSI equipment connected to
your computer. Each device must have a unique ID number
(the computer itself has the ID number 7, and the internal
hard disk has the number 0). Also check that the chain of
devices is terminated properly. (See the chapter on enhancing
your computer system for information about setting SCSI ID
numbers and checking termination.)
Using Disk First Aid
You can use the Disk First Aid program to test floppy disks
and hard disks that may have been damaged. Disk First Aid
also repairs some types of disk damage. Disk First Aid is on the
system software disk labeled
Disk Tools.
1. Insert Disk Tools and start up your computer.
2. Open the Disk First Aid icon.
114 Chapter 7: Using Disks