You can turn on Selective Sync by going to Dropbox's menu bar icon on your Mac and clicking the Settings button; from there, select the Account tab and click on Change Settings under Selective Sync. Folder choice is granular: You can sync entire top-level Dropbox folders (of which my Macworld work is one), but you can also choose subfolders (say, "Observatory Project" inside "Art Projects") within Dropbox without having to download the entire containing folder.
Selective Sync is especially important for me because of my MacBook's capacity limitations. I don't even think my 88GB Dropbox folder would fit on my MacBook Air! Instead, I let my iMac take care of my master file collection there, and sync only what I need to work on at any given time. If I want to access something that's not readily available on my MacBook Air, all I have to do is go into Selective Sync preferences and add it, or visit Dropbox.com on the Web.
Keep your applications in tune
The biggest hurdle in keeping your data square across your Macs may be the data itself, but I had one more problem to tackle: What about the data in my programs? To give you an example: I started working on this article on my MacBook Air, but while doing so, I also opened up a few unsaved documents in BBEdit to jot down a quick outline, along with some links. The article I easily saved to my Dropbox folder and reopened on my iMac, but what about those unsaved notes?
Well, Dropbox isn't just good for saving static files — some apps, like BBEdit, let you sync application support files within the folder, too. The key is in your computer's Application Support folder, hidden within ~/Library: Your programs use information from that folder to save your preferences, load any unsaved data, and the like. If you want to sync your program states, just drag your BBEdit support folder from ~/Library/Application Support to a new Application Support folder within Dropbox.
Now, whenever I open up BBEdit on my iMac after using it on my MacBook Air, all my unsaved documents are present and ready to be worked with or discarded.
Other apps offer Dropbox sync support, too: AgileBits's 1Password offers a Dropbox sync button directly within the application, while programs like launch manager Alfred can store a single preference file on the service.
And I haven't even mentioned the programs that have their own sync solutions, or applications that use Apple's iCloud service. As we start to move away from a static, singular file system, your programs are becoming more dependent on cloud services — making sure that your app looks like your app, no matter what Mac you open it on.
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