Really you can use whichever cloud storage provider you want, but I've been throwing tens of thousands of files at Dropbox at once, and it's extremely solid. You'll also want its version history.
This is where you'll be saving all your Reaper files from now on. Install it, create a music directory, organise it how you'd like, move all your existing projects into here and start saving all your new ones in here.
Set Up Autosave in Reaper.
Navigate to Options > Preferences > Project
Set the following:
See that "(not recommended)" after "Save to project file"? This is part of why we set up Dropbox. The recommended method is to create a new file for each backup, but this leaves an absolute mess to deal with and mantain. Instead, we use Dropbox's built in version control, which holds all copies for 30 days. Right click on any file in your Dropbox folder, select Version History to see all the times you've saved the file, and you can restore from a previous save.
If you need longer than 30 days recovery you can pony up for the year-long version, or use the other Reaper option of a new file each time.
Change Reaper's Default Recording Path.
Reaper default's to saving in a folder under Documents for any takes in unsaved projects. So if you often open a new project and quickly jot down an idea before saving, this folder is going to be big. Change the default path so all files are protected.
Navigate to Options > Preferences > Paths
Note the current location, then set the following path to a folder under your Dropbox folder
Set Reaper to Record Into Subdirectories
We want Reaper to where possible save recordings into a folder under each project directory. This only works after the project has a file and directory so always save before doing any recording if possible.
Navigate to File > Project Settings > Media
Set the path to 'Media' and save it as the default for all new projects
Update any existing projects you're working on
Move Existing Media.
Now let's sort out the existing saved files.
Navigate to the old location. it should be under your Documents folder. Confirm you can see 'Reaper Media' and its got some takes in it. If it's empty, you can skip this section
If it's on the same drive as your Dropbox folder, we can move the folder without actually transferring the data (the OS will just readdress it). If it's on a different drive, make sure there's enough space in the destination. We're cutting and pasting here so if something goes wrong there's always the risk of losing data. Back it up before the move if you want to be super safe
Right click on the existing Reaper media folder and select 'Cut'
Right click inside your destination Dropbox location and select 'paste'
If the move was on the same drive and successful, it should have been instantaneous
Now we'll create a symbolic link from the old location to the new. Open up command prompt as admin
mklink /D <path to old location using quotes> <path to new location using quotes>
And press enter. You should see a new folder with a shortcut icon in the old location. if you double click it, it'll show files from the new location
Move Existing Files from Reaper Media to Subdirectories (Optional but Recommended)
For any existing projects you open up, you should force a move of the files to keep things together, manageable and clean.
Open an existing project
Select File > Save As
Tick 'Move all media into project directory'
Accept the overwrite because we want it to stay the same project file
Back Up Reaper Settings (Optional but Recommended)
Reaper stores settings in a folder under roaming appdata: %appdata%\Reaper. This changes only as often as you change your settings, so can be backed up less often than projects. The cleanest way is to reinstall the portable version, as this will run out of a single folder that you can put in Dropbox. However if, like me, you're running the full version, you can manually copy that folder to Dropbox intermittently, set up a script to do it, or set up a script and a scheduled task.
I'd suggest creating multiple dated copies for this backup as it won't be kept by the free tier of Dropbox's version control.
The script will be different depending on your archiving software. An example for WinRar, written in PowerShell (and wrapped with a batch file or link to bypass the execution policy for just this script) might look like this:
The action for the scheduled task would be the contents of the batch file, with -windowstyle minimized added, and lines 8 and 9 removed from the PowerShell script.
Hopefully you've implemented this early enough to save you serious heartache. If not, set it up now and know that great melodies will always come back to you.