MobileMe is best known for its highly visible services such as email and iDisk, but one of its most impressive talents is keeping many kinds of data in sync among multiple Macs and iOS devices. Because Apple’s servers function as both mediator and online storage for all your synchronised data, you can also access your information in a web browser at

Before you start experimenting with changes in your syncing setup, back up your calendar and contact data, which are especially vulnerable to accidental change or deletion. If you already use Time Machine or another backup utility, you’re all set. Otherwise, to back up the contents of OS X’s Address Book, open Address Book (in /Applications), and choose File > Export > Address Book Archive. To back up your iCal calendars, open iCal (in /Applications), choose File > Export > iCal Archive. If you’re using an iPhone, iPad or iPod touch, sync it before proceeding.

Basic setup: On a Mac, open the MobileMe system preference and sign in if necessary. Next, click the Sync tab and select the Synchronise With MobileMe checkbox. The pop-up menu provides several options for synchronisation frequency. The default choice, Automatically, is what Apple recommends (as do we). For most data types, this setting means that when data changes on your Mac, it’s pushed to Apple’s servers immediately, and when changes are made elsewhere, they’re retrieved from Apple’s servers quickly.

You’ll also need to select the checkbox for each type of data you want to sync. Remember that it never hurts to activate and test these one at a time. Finally, make sure Show Status In Menu Bar is selected; doing so displays the Sync menu (two arrows forming a circle) in your menu bar, which will come in handy later on.

On the Sync tab of the MobileMe system preference, you can turn on syncing for various types of data and adjust synchronisation options

On an iOS 4 device, if you don’t already have a MobileMe account set up, tap Settings > Mail, Contacts, Calendars > Add Account > MobileMe, enter your credentials, and tap Next. After iOS verifies your information, it displays a series of services you can turn on or off, but leave things set at their defaults for now, and tap Done.

Email: MobileMe addresses the problem of syncing email in two ways. First, your MobileMe email account uses IMAP, so the mail server contains a master copy of all your messages and mailboxes – including sent and filed email.

Second, MobileMe lets you sync the settings for all your accounts (not just MobileMe) between devices, although it doesn’t sync the messages themselves. If each of your accounts is of the IMAP or Microsoft Exchange variety, syncing is simple because MobileMe handles syncing account settings, while Mail syncs the messages themselves with the servers.

On the Sync tab of the MobileMe system preference are two checkboxes to do with Mail. When you select Mail Accounts on two or more Macs, OS X synchronises the basic settings (such as server addresses and email addresses) for all your email accounts between them. This setting does not sync your passwords, though; if you store your Mail passwords in your Keychain, you can select Keychains, separately, to sync those too. The result is that for IMAP and Exchange accounts, both Macs will download and display identical copies of the messages from the respective servers. (Messages stored locally in On My Mac mailboxes don’t sync via MobileMe.)

When you select Mail Rules, Signatures, and Smart Mailboxes, those portions of Mail’s preferences sync between the Macs. This is usually helpful, but if two Macs have different sets of active accounts or local mailboxes, some of your items may not work correctly.

Although MobileMe syncs Mail settings between Macs, it doesn’t sync Mail settings to your iOS devices; iTunes does that job. Connect your iOS device, select it in the Devices section of iTunes’ Source list, then click the Info tab. Select Sync Mail Accounts, along with each email account on your Mac for which you’d like the settings transferred to your iOS device. Then click Sync.

Once you’ve done this, go to Settings > Mail, Contacts, Calendars on your iOS device, tap your MobileMe account, and make sure that the Mail switch is set to On. Repeat this procedure for each server-based account you want to sync.

Make sure Fetch New Data is set to Push if you want data to update immediately on your iPhone when they change on another device.

Note that on iOS devices, Mail doesn’t automatically download all the messages in all your mailboxes; it downloads the contents of your Inboxes (up to the limit specified in Settings > Mail, Contacts, Calendars > Show), and then it downloads the limited contents of any individual mailbox when you select it.

Contacts: To sync your contacts via MobileMe, make sure the Contacts checkbox is selected in the Sync tab of the MobileMe system preference. OS X then syncs all the local (On My Mac) contacts between your Mac(s) and Apple’s servers. Contacts on LDAP servers, or in CardDAV or Exchange 2007 accounts don’t sync via MobileMe; you must set up these accounts separately on each device.

To enable contact syncing on an iOS device, tap Settings > Mail, Contacts, Calendars, tap your MobileMe account and make sure Contacts is set to On. To sync with Exchange accounts, tap their names and ensure that Contacts is on.

Calendars: MobileMe can sync the calendars you created on your Mac(s) in iCal with Apple’s servers to other Macs, and to your iOS devices. Apple recently revised MobileMe to use the industry-standard CalDAV protocol for syncing calendar data, but you’ll have to activate it first. Sign in to the MobileMe website, go to the Calendar application and click Upgrade Now.
On each Mac, make sure that Calendars is selected on the Sync tab of the MobileMe system preference. Note that MobileMe syncs only local (On My Mac) calendars and read-only calendars to which you’ve subscribed. If you’ve set up any server-based calendar accounts, such as CalDAV or Exchange 2007, these won’t sync via MobileMe.

To sync calendars on an iOS 4 device, go to Settings > Mail, Contacts, Calendars > yourMobileMe account and make sure Calendars is turned on. You may see a prompt asking how to treat existing calendar data. If so, Merge With MobileMe is the correct choice.

Bookmarks: If you select the Bookmarks checkbox on the Sync tab of the MobileMe system preference on each of your Macs, OS X syncs Safari’s bookmarks between them; no further configuration is needed. On an iOS device, tap Settings > Mail, Contacts, Calendars  > yourMobileMe account and make sure the Bookmarks switch is set to On.

Troubleshooting: Because syncing is complex, problems can occur. Here are some quick tips:

  • In OS X, MobileMe can notify you if a pending sync would change more than a small amount of data. Open the MobileMe system preference, click the Sync tab, and click Advanced. At the bottom, select Show Alert When X Of The Data On This Computer Will Be Changed, and choose an amount. (The More Than 25% option is advisable.)
  • In certain situations – such as when one of your devices is offline for a while – you could end up changing a particular item on two different devices in between syncs. When this happens, OS X doesn’t know what to do and presents a Conflict Resolver dialog box. Click Review Now to see which items are in conflict and click the one you want to use.
  • If your data has become badly out of sync, you can reset it – replace the synced data on your Mac with the copy that’s on the MobileMe servers or vice-versa. To do this, open the MobileMe system preference, click the Sync tab, and click Advanced. At the bottom of the dialog box, click Reset Sync Data. Choose the type of data to replace (or choose All Sync Info, to replace everything) and then choose the direction the replacement should occur.
  • The Sync menu that you enabled earlier lets you force a sync to occur immediately (Sync Now) or stop a sync in progress (Cancel Sync). Hold down the Option key while clicking the menu, and you’ll see additional commands. For problems not solved by resetting sync data, choose Reset Sync Services. This leaves your data intact but erases your sync history, which can solve certain tricky syncing problems.