i have a white screen on start up

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It means that OS X can't start. As you are using OS X Mountain Lion, try starting into OS X Recovery, holding Command and R keys while your Mac is starting.

Then, open Disk Utility, select "Macintosh HD" on the sidebar and verify the disk. If it's OK, close Disk Utility and reinstall OS X. If it's damaged, repair it and restart. If the MacBook Pro can't start after repairing the hard drive, reinstall OS X

If the hard drive has not failed, then try this:

Reinstalling Lion/Mountain Lion Without Erasing the Drive

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.

Reinstall Lion/Mountain Lion: Select Reinstall Lion/Mountain Lion and click on the Continue button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

Restart your Mac and hold down the Command key and the R key (Command-R), and keep holding them until the Apple icon appears.

When the startup is completed you will see a screen with four choices. Select Disk Utility and click Continue. In the new window, on the left, select the name of your drive, then click on Repair Drive.

When the repair is done, quit DU and restart.

Take each of these steps that you haven't already tried. Stop when the problem is resolved.

How to back up and restore your files

b. If you have access to a working Mac, and both it and the non-working Mac have FireWire or Thunderbolt ports, boot the non-working Mac in target disk mode by holding down the key combination command-T at the startup chime. Connect the two Macs with a FireWire or Thunderbolt cable. The internal drive of the machine running in target mode will mount as an external drive on the other machine. Copy the data to another drive. This technique won't work with USB, Ethernet, Wi-Fi, or Bluetooth.

How to use and troubleshoot FireWire target disk mode

c. If the internal drive of the non-working Mac is user-replaceable, remove it and mount it in an external enclosure or drive dock. Use another Mac to copy the data.

Step 2

Press and hold the power button until the power shuts off. Disconnect all wired peripherals except those needed to boot, and remove all aftermarket expansion cards. Use a different keyboard and/or mouse, if those devices are wired. If you can boot now, one of the devices you disconnected, or a combination of them, is causing the problem. Finding out which one is a process of elimination.

If you've booted from an external storage device, make sure that your internal boot volume is selected in the Startup Disk pane of System Preferences.

Step 3

Boot in safe mode.* The instructions provided by Apple are as follows:

• Shut down your computer, wait 30 seconds, and then hold down the shift key while pressing the power button.
• When you see the gray Apple logo, release the shift key.
• If you are prompted to log in, type your password, and then hold down the shift key again as you click Log in.

Safe mode is much slower to boot and run than normal, and some things won’t work at all, including wireless networking on certain Macs.

The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

*Note: If FileVault is enabled, or if a firmware password is set, or if the boot volume is a software RAID, you can’t boot in safe mode. Post for further instructions.

When you boot in safe mode, it's normal to see a dark gray progress bar on a light gray background. If the progress bar gets stuck for more than a few minutes, or if the system shuts down automatically while the progress bar is displayed, your boot volume is damaged and the drive is probably malfunctioning. In that case, go to step 5.

If you can boot and log in now, empty the Trash, and then open the Finder Info window on your boot volume ("Macintosh HD," unless you gave it a different name.) Check that you have at least 9 GB of available space, as shown in the window. If you don't, copy as many files as necessary to another volume (not another folder on the same volume) and delete the originals. Deletion isn't complete until you empty the Trash again. Do this until the available space is more than 9 GB. Then reboot as usual (i.e., not in safe mode.)

If the boot process hangs again, the problem is likely caused by a third-party system modification that you installed. Post for further instructions.

Step 4

Sometimes a boot failure can be resolved by resetting the NVRAM.

Step 5

Launch Disk Utility in Recovery mode (see step 1.) Select your startup volume, then run Repair Disk. If any problems are found, repeat until clear. If Disk Utility reports that the volume can't be repaired, the drive has malfunctioned and should be replaced. You might choose to tolerate one such malfunction in the life of the drive. In that case, erase the volume and restore from a backup. If the same thing ever happens again, replace the drive immediately.

This is one of the rare situations in which you should also run Repair Permissions, ignoring the false warnings it may produce. Look for the line "Permissions repair complete" at the end of the output. Then reboot as usual.

Step 6

Boot into Recovery again. When the OS X Utilities screen appears, follow the prompts to reinstall the OS. If your Mac was upgraded from an older version of OS X, you’ll need the Apple ID and password you used to upgrade.

Note: You need an always-on Ethernet or Wi-Fi connection to the Internet to use Recovery. It won’t work with USB or PPPoE modems, or with proxy servers, or with networks that require a certificate for authentication.

Step 7

Repeat step 6, but this time erase the boot volume in Disk Utility before installing. The system should automatically reboot into the Setup Assistant. Follow the prompts to transfer your data from a backup.

Step 8

A dead logic-board battery in a Mac Pro can cause a gray screen at boot. Typically the boot failure will be preceded by loss of the startup disk and system clock settings. See the user manual for replacement instructions.

Step 9

If you get this far, you're probably dealing with a hardware fault. Make a "Genius" appointment at an Apple Store to have the machine tested.

Hello Davis, I have the same problem but for meI clicked by mistake on no access on my start up disk (get info) and I have no longer access on my computer, I switched off and on, I don't see where to put the password, its appeared only white screen. any suggestion to enter in computer and change to have the access?

Thank you

<Email Edited by Host>

Hi!

Today, I encountered the same problem. I started restarting and starting in guest - mode, that didn't work. After this, I reset my PRAM, wich didn't work either. Then, I booted up in Safe Mode, to discover that my hard drive had only 1 GB available. So I backed up some files, got rip of a lot of temp-files, deleted some files, and tried restarting again. My Macbook is now running without problems.

I had this problem, and it was because the SSD harddisk was full. Going into safe boot mode and clearing the trash per Linc's instruction in step 3 solved it.

OK, so I came across this discussion and followed suggestions. Yours well outlined, Thanks.

I can boot in safe mode and have deleted files to >10% free space (14Gb free of 100Gb)

Target disc mode works fine and I copied what I wanted.

Disc repair says Disc OK, did repair permissions. (ran from safe mode and as target drive)

Still get white screen when booting AND white screen when I try to boot from CD.

Does this point to hardware logic board problem?

this is an old MacBookPro2,2 so its time has probably come.

did i mention that prior to start up there are diagonal bars/lines across screen? these do go away in safe mode.

I tried all of the solutions here using the one mac keyboard I have from my second (working) imac. After failing with all, and with every solution I could find online short of taking it apart, I reattached my mac keyboard to the working imac and put the windows keyboard I had been using back on the iMac that booted to a white screen. Out of curiosity I went to the windows site and got the windows key equivalents for the mac keys and tried the combination for shutdown (I was tired of pushing the power key to shut it down after days of trying to solve the boot up problem) I pushed control U U and rather than shutting down the recovery screen appeared. After doing a very happy dance I decided to post this. May or may not help.

Linc,

Thanks for the insight, control, P, R….utilities….verify disk, repair permission

Reboot & Voila, it works once again. Mahalo once again.

Gene

This was extremely helpful! Laptop started working right away! Thank you!!