Google Drive works on Android, iOS, Windows, OSX
So does Dropbox, a similar cloud-drive service.
When you set up your devices on the same account, the cloud drives will synchronize across your devices automatically.
On desktops, the files are actually synchronized on your hard drive.
In the case of the mobile devices, the documents will be downloaded on the fly.
I'm not sure what you mean by "Is there anyway you can turn a external storage device into a wireless cloud device?"
When you install a cloud service, it creates a virtual drive, that you can save, modify and delete files on. It behaves just like a hard drive or a USB drive.
You can configure Dropbox and Google Drive to synchronize to a USB drive, if that's what you're asking.
https://productforums.google.com/forum/#!topic/drive/qgrwOfr-GfQDropbox you can just change the sync directory on the fly.