If the Macintosh has been started up with the System 7 "Disk Tools" floppy
disk and Disk First Aid cannot repair a hard drive's directory, the drive
must be reinitialized with an appropriate formatting utility. Apple hard
drives are initialized with Apple's HD SC Setup utility. Third-party hard
drives require System 7-compatible initializing/formatting software
supplied by the vendor.
IMPORTANT: Before initializing the drive, make at least one backup of the
drive using System 7-compatible backup software, otherwise you will lose
everything on the drive.