Q: SSD Filling Up

Firstly, only data from your iDevices to to iCloud. Do you mean iCloud Drive? If so it's because a copy is maintained on your startup drive. Dropbox maintains your data in your DropBox folder on your hard drive. If you are filling up your SSD it's because you are storing stuff on it unnecessarily. Either you can delete it or you need to use an external drive to save stuff you rarely use.

Freeing Up Space on The Hard Drive

  1. See Lion/Mountain Lion/Mavericks' Storage Display.

  2. You can remove data from your Home folder except for the /Home/Library/ folder.

  3. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.

  4. Also see Freeing space on your Mac OS X startup disk.

  5. See Where did my Disk Space go?.

  6. See The Storage Display.

You must Empty the Trash in order to recover the space they occupied on the hard drive.

You should consider replacing the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.

Try using OmniDiskSweeper 1.8 or GrandPerspective to search your drive for large files and where they are located.

No. Your iCloud Drive is a space on Apple's servers. If you need more, then you need to buy more - up to 200GB. iCloud Drive is not a backup solution for your computer.