What does it mean when a folder with a question mark in it flashes on a white screen

There are four general causes of this issue:

1. The computer's PRAM no longer contains a valid startup disk setting when there aren't any problems with the disk itself. This can be checked for by pressing the Option key and seeing if the drive appears.

2. The internal drive's directory structure has become damaged. This requires usage of an alternate bootable system to perform the repair.

3. Critical system files have been deleted. This requires usage of an alternate bootable system to reinstall them.

4. The internal drive has died or become unplugged. This is the most likely case if the computer took a sharp impact or there are unusual sounds coming from the hard drive's location.

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A flashing question mark appears when you start your Mac

any chance it was not shutdown gracefully?

power went out at some point?

whatever it is or was not, would hope a few things:

that you are not running Leopard still;

that you have bootable backup clone(s) of your drive(s) and system;

TimeMachine, not that great back in 10.5.x days but better now

Every Mac Pro can run Lion or later which is a good idea, keep Snow leopard and dual boot if you still require use of PowerPC apps

Boot from DVD or Lion Recovery or a clone of the system and repair;

Follow up with Safe Boot.

Startup Manager: How to select a startup volume

http://support.apple.com/kb/HT131

Troubleshooting: My computer won't turn on

https://support.apple.com/kb/TS1367

General purpose Mac troubleshooting guide:

Isolating issues in Mac OS X

Creating a temporary user to isolate user-specific problems:

Isolating an issue by using another user account

Identifying resource hogs and other tips:

Using Activity Monitor to read System Memory and determine how much RAM is being used

Starting the computer in "safe mode":

Mac OS X: What is Safe Boot, Safe Mode?

To identify potential hardware problems:

Apple Hardware Test

General Mac maintenance:

Tips to keep your Mac in top form

Using Cloning as a Backup Strategy

Carbon Copy Cloner 3.4.1

http://www.bombich.com/software/updates/ccc-3.5.html

OS X Lion Install to Different Drive

How to create an OS X Lion installation disc MacFixIt

Migration Assistant Update for Mac OS X Snow Leopard