Just a heads up people.... A major bug in an Apple operating system is an unusual event, but they do happen, and the recent bug uncovered in the new 10.6 (Snow Leopard) version of OS X is about as major as bugs get. So what, is this bug, what does it do, why, and most importantly how can you protect yourself against the effects of it? In short, when a user logs out of a guest account the Mac can erase the entire contents of the users home folder, resulting in complete loss of all the data in this account, usually all a users documents, pictures, music. Applications are not directly effected other than the loss of preferences from the user account. But why would your Mac do this? A guest account is a special sort of account, introduced with OS X 10.5 (Leopard). It allows guests to use a Mac to browse the internet, check mail or perform other tasks. when the guest logs out of the account, all data is deleted, protecting their privacy and leaving the account clean and tidy for the next guest user. Guest accounts are created but NOT enabled by default in 10.5, but many users will have enabled them. Guest account data is not only deleted but securely deleted. This makes recovery, even by tools such as Data Rescue II impossible. Users have been reporting that logging into and out of a guest account results in this behaviour being mistakenly applied to the main user account with disasterous results. A few conditions do have to be met for this bug to rear its head. The guest account needs to have been created in version 10.5. Owners of brand new macs shipped with 10.6 are not effected, however the number of people with guest accounts created in 10.5 then upgraded to 10.6 is large. So what can you do to protect your self? If you think your guest account may have been created in 10.5 the safest course of action is to disable it via the system preferences. Open system preferences, via the apple menu or dock and select accounts. Unlock the accounts pane using the padlock in the bottom left corner using your admin password to confirm. Select the guest account. To the right you will see an option to enable or disable the account. Make sure it is unchecked to disable it. Re-click the padlock to lock the account changes. It is likely that Apple will issue a fix for this bug via Software update i the near future, so either check manually for updates via the apple menu or set software update to check more often for updates. When the patch is made available download and install it. Of course, there is one way to protect yourself from this bug, even if the worst should happen, and that is of course to ensure you are regularly backed up. Time Machine ensures that should the bug strike you, you can at least recover your data from a recent backup. To reiterate one of my favourite maxims (courtesy of Alex Lindsay of the Pixel Corps), data does not exist unless it exists in two places simultaneously. The best defence is a good defence.