What you are asking for sounds a bit overkill for SVN, but it is not outside the realms of possibility.
The main question to ask is do you really need the version control abilities of SVN or could you live without it.
With TortoiseSVN you could potentially create a local "file" based repository and then check out copies from that repository to your main hard drive and external drive, you would then need to update both copies on a semi-regular basis.(I'll elaborate on this shortly)
A good tutorial on creating a local SVN repository is in the TortoiseSVN documentation section titled Creating The Repository With TortoiseSVN. The basic procedure is (after installing TortoiseSVN) as follows:
Create a directory where you want to actually store the repository data (Say C:\SVNRepo) and go into that directory.
Right-click anywhere in that Explorer window, go into the TortoiseSVN dropdown and select "Create repository here"
Elsewhere in your system (say your desktop or external hard drive or both) create another directory called something like SVNCheckout
In that directory right click again and this time select the SVN Checkout option and where it asks for the repository location enter the path to where you created the actual repository, You'll need to put file:// at the start of the path and substitute all "\"'es with a "/". In the end it should look something like file://C:/SVNRepo
You will now be able to manage all your files that you want to keep synchronised in the normal SVN way. You'll have to remember to commit any changed files that you want to then be able to transfer between checkout locations.
What this does mean though is that for any files you manage via SVN you will have at least two copies on your local system, one in the repository store, and one in the checkout directories.
Alternatively if all you want is a backup copy and don't care about version control as much I would recommend a file syncronisation tool such as SyncbackSE which is free and can be scheduled to regularly update files between both copies.
As a side note I believe that Syncback can be configured to check if the destination device actually exists to before synchronisation (or there are other simple file synchronisation software that can be configured to) so in order to
There it is, in the SyncbackSE feature list:
Run a profile automatically when external media is connected, e.g. a USB key is plugged in
As well as
File versioning so you can roll-back changes or deletions
Sadly though, these features are missing from their freeware versions and are only in their payware solutions.
There are some alternatives for file/folder synchronisation in this review: http://www.techsupportalert.com/best-free-folder-synchronization-utility.htm
