I have 3 macs all running ElCapitan. Two are iMac desktops and one is a macbook air. They are all fairly new and all were purchased with 500 GB Flash drives (not fusion). All three computers are running the same programs and apps. Both the iMacs have lots of storage still available (about 250GB each). But the Mac Book Air is almost out of storage. I'm getting warnings on the Air that dropbox can't sync because I don't have enough space. There is only about 18 GB of storage available on the Air. But it doesn't make sense, because the macbook air doesn't have anything additionally installed or any files/programs more than what is on the two iMacs. Is there an explanation for this? Thank you!