git-annex might be a good candidate here. You can think of it as a kind of "open-source Dropbox and more". It can work locally, via network, with online media.

At a first look it might seem a bit "confusing and hard to set up" – but luckily there's git-annex assistant which makes things easy (just look for that at Youtube for some nice presentations). For your case, I'd recommend using that to set up two "annexes": One is the download directory on your local harddisk, and the other is your "external drive". Now, everything you drop in one place, is easily "known" on the other – though by default not "physically there". For the latter, you simply declare the entire "local harddisk annex" as "preferred content" in your "external annex" – which then would take care to automatically make the content itself available there (instead of just the metadata).

^{git-annex assistant guides you through the setup process in easy steps (click images for larger variants)}

The entire process is not usually triggered immediately, but might have a short delay (the daemon checks in intervals AFAIK). git-annex works fine on Linux and Android (not sure about other systems, but if I remember correctly, also on Mac and Windows).

^{a git-annex assistant demo screen: matches your situation with the USB stick :)}

Some additional links which might prove useful:

• How can I install git-annex assistant (on our sister-site AskUbuntu)
• file manager integration
• git-annex projects at Github (addons, plugins, manuals, you-name-it)
• Managing a large number of files archived on many pieces of read-only medium (E.G. DVDs)
• Managing Backups With Git-annex
• backends and special remotes (your external harddrive might fall into the latter category)

git-annex might be a good candidate here. You can think of it as a kind of "open-source Dropbox and more". It can work locally, via network, with online media.

At a first look it might seem a bit "confusing and hard to set up" – but luckily there's git-annex assistant which makes things easy (just look for that at Youtube for some nice presentations). For your case, I'd recommend using that to set up two "annexes": One is the download directory on your local harddisk, and the other is your "external drive". Now, everything you drop in one place, is easily "known" on the other – though by default not "physically there". For the latter, you simply declare the entire "local harddisk annex" as "preferred content" in your "external annex" – which then would take care to automatically make the content itself available there (instead of just the metadata).

^{git-annex assistant guides you through the setup process in easy steps (click images for larger variants)}

The entire process is not usually triggered immediately, but might have a short delay (the daemon checks in intervals AFAIK). git-annex works fine on Linux and Android (not sure about other systems, but if I remember correctly, also on Mac and Windows).

^{a git-annex assistant demo screen: matches your situation with the USB stick :)}

Some additional links which might prove useful:

• How can I install git-annex assistant (on our sister-site AskUbuntu)
• file manager integration
• git-annex projects at Github (addons, plugins, manuals, you-name-it)
• Managing a large number of files archived on many pieces of read-only medium (E.G. DVDs)
• Managing Backups With Git-annex
• backends and special remotes (your external harddrive might fall into the latter category)