One of my Macs running OS X Leopard was panicking after I upgraded its memory; while waking from hibernate, I’d get the grey curtain of death and the multilingual “reboot this computer” message panel. If it happens to you too, you’ll be glad to know there’s a solution.
When a portable mac goes to sleep, a feature called SafeSleep causes
the computer’s active memory to be written to a sleep image file (at
/var/vm/sleepimage) before the machine’s power light starts pulsing.
The battery keeps the active memory intact while the mac is asleep.
If the battery is removed, the computer is considered to be in
hibernation; when the power is later restored, the operating system’s
in-memory state will be restored from the sleep image file.
If the machine’s memory gets upgraded, the sleep image file will be automatically regenerated to match the size of the new physical memory. In my case, this regeneration seemed to stop my machine from waking up properly from hibernation.
A little Googling led me to this page, which mentions that there are distinct “encrypted” and “unencrypted” hibernate modes, that the default mode is “encrypted”, and that problems can occur when secure virtual memory is hibernated into a sleep image for encrypted hibernate mode.
I had noticed after my Leopard upgrade that in the Security preferences pane, the “secure virtual memory” option was enabled; I don’t know what its setting was prior to the upgrade, but following the advice in the above page I executed the following command in a terminal to set the system’s hibernate mode to 7:
After this, my Mac will happily wake from sleep.
An alternate solution might have been to disable secure virtual memory. If I do so in future, I expect that I would need to set the hibernate mode to its default value of 3 instead.
Other hibernation mode tricks
Setting the hibernate mode to 0 will disable hibernation, which will make your portable Mac skip the creation of sleep image files and so fall asleep faster, at the expense of being able to recover your desktop state and open documents if you remove the battery or allow it to run completely flat while asleep.
Desktop Macs, such as Mac Minis, use a hibernation mode of 0 by default. I hear you can also set their hibernation mode to 3 or 7 to make them create sleep images, and then simply power them off when you go on vacation, after putting them to sleep, of course.