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If the Dropbox location on your Macbook Air is on the startup disk then you will need to change that location to another disk to allow the sync to continue and then free up some space on your startup disk. Once you have done this, the sync should continue normally again and iPhoto should open up ok.
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Ok, then you need to try the same thing on the iMac - change the Dropbox location to another disk if there is insufficient space, then try syncing again.
If you search in Finder or Spotlight for Dropbox it will take you to the Dropbox location.
Did you find it?
Is it working ok now?