Check all the Connections. Make sure that the external drive is connected properly to the Macintosh. Make sure the drive is on and plugged into a power outlet. Check that all cables are firmly connected. Restart.

Disconnect Any External SCSI Devices and Restart. Disconnect the SCSI cable from the back of your Mac. If this succeeds in getting your Mac to start up, you probably have either a SCSI connection problem or a hardware problem with one or more of your external SCSI devices.

Most hard drives have one or two indicator lights on the front of the unit (the light is built into the front panel of the Macintosh if it is an internal drive). If there are two lights, one usually indicates that the drive is on, while the other only goes on if the drive is being accessed (reading or writing). If there is only one light, it is usually an access light. In either case, the light should go on - at least intermittently - at startup. If it does not, it means that the drive is not functioning (assuming that the light bulb itself is working okay!), and a likely hardware repair looms ahead.

Restart Again with the Same Disk as a Startup Disk. Hopefully, the problem will disappear, in which case your troubles may be over. If it returns later, however, you may have an intermittent problem.

Restart with an Alternate Startup Disk. If the previous restart did not work, try restarting with an alternate disk, preferably a floppy disk. If this succeeds, continue to the next step. By the way, if you are starting up from a floppy disk, you can still change settings of some control panels on your hard disk. For example, you could change the Startup Disk setting from an external drive (if that is its current setting) back to an internal drive for your next restart.

If you can get the Macintosh to start up with an alternate startup disk, check to see if the problem disk has mounted and is now present as a secondary disk (that is, its icon is shown below the startup disk icon on the Finder's desktop). Most likely it will be there. If so, check for the following problems (using the problem disk as the startup disk, restart the Macintosh after each attempted solution to see if the problem is solved).

Make Sure a System Folder Is on the Problem Disk: Presumably you already know that a System Folder is there. But just in case, check anyway. If there isn't one already there, install a System Folder and start again.

Make Sure There Is Only One System Folder on the Disk: Multiple System Folders (more than one copy of the System file and/or Finder) on your startup disk are not likely to cause startup problems, especially if you are using System 6.0.7 or later. Still, to be safe, it is good practice to delete all but the intended startup System Folder from your disk. If you are unsure whether additional System Folders are present, you can use the Finder's Find command and search for "Finder" or "System."

Replace the System and the Finder: If the previous steps have not worked, the System file, the Finder and/or the Update file may be damaged. TechTool and TechTool Pro are useful for checking for damage to these files. Otherwise, to be safe, just replace the files. You might also try replacing the Finder Preferences file.

If your startup disk is divided into partitions, only one of those partitions can normally act as the startup disk. If you place a System Folder on another partition and try to select it as the startup disk (via the Startup Disk control panel,) it will not work. In some cases (depending on your particular disk driver and especially if the only available System Folder is on the partition that cannot act as the startup disk), the result may be that you get the cycling happy Mac. If this happens, follow the steps listed here.

Select a Different Startup Disk: First, start up from a floppy startup disk. Then, if needed, install a System Folder on the partition that can act as a startup disk. Next, select that disk (or any other valid startup volume) as the startup disk from the Startup Disk control panel. (If you had used System Picker originally, you may need to use it again to select a new startup disk.) Restart.

Update Your Disk Driver: Switching to a different disk driver may eliminate this cycling happy-Mac problem altogether, allowing you to start up from the previously problematic partition. In general, however, you will keep these and other related problems to a minimum by using the disk driver from the utility that was used to format your disk and by using the same utility for all of your mounted volumes.

An incorrect or out-of-date driver can prevent startup even if the driver is on a secondary hard drive rather than the startup drive itself. All mounted drives should have correct drivers.

Parameter RAM (or PRAM) is a special type of memory that is preserved by a battery inside the Macintosh even after the computer is shut off. Some of the data stored in PRAM relate especially to SCSI connections. If the data stored in PRAM become corrupted, they can prevent a hard disk from mounting properly. Zapping the PRAM restores the data to their default value and should solve this problem.

There have been reports that if the battery installed on the logic board of your Mac (the one that maintains the PRAM settings etc.) goes dead, you may lose not only your PRAM settings but the ability to start up your Mac at all. Fortunately, replacing the battery fixes the problem, so no major repair is needed.

On some Macs, a dead battery may result in a normal startup except that the monitor display remains black. If this happens, try zapping your PRAM. If the video comes back, you probably need to replace the battery.

If none of the preceding solutions worked, or if the problem disk did not mount when starting up with the alternate disk - or if you could not start up at all when using an alternate disk - try the following steps.

"Repair" and Defragment the Disk: If you get the blinking question-mark icon as soon as you restart after a system crash (or some other improper shutdown), you may have something called the Disk Check bug. What is happening is that the Macintosh's startup diagnostics are mistakenly reporting a damaged disk, even though the disk is fine.

To fix this, use a repair utility, such as Disk Doctor (Norton Utilities), to "repair" and mount the disk. This disk will now work normally.

To prevent this from happening in the future, defragment your disk. Continue to defragment it on a regular basis.

To see if you are even susceptible to this particular bug, run a freeware utility called Disk Bug Checker. Supposedly, this problem has been fixed altogether in System 7.5 and subsequent revisions.