I've been saving my projects on a USB drive because, when I first got my computer and USB 3 was basically brand new and I just started doing that and haven't changed over.  
 
Anyway, I'm recording an album of original material with my friend.  Just realized that there's almost no space left.  These tracks are basically "scratch tracks" for the drummer to do the real drum tracks.... once those are done, we're going to get rid of most of these tracks and do the final tracks, except for maybe a few here and there.  Right now, there are Addictive Drums patterns there just to have something until the real drums are recorded. 
 
Is there ANYTHING I can do to create space WITHOUT deleting tracks?  Obviously, I'm going to delete the Addictive Drums patterns that I programmed so the real drummer can do his parts.  Will that create enough space?  I thought about maybe deleting some of the panned rhythm parts and centering one of them.... I'd rather not do that if I don't have to.  Any ideas?  What about plugins?  Do plugins take space?

Maybe get a larger USB drive and just transfer the files to it? If you now have a hard drive with more space, trasfer the files over and go from there. Otherwise if you're out of space you're done, unless there other non related things you can delete.

Ok, so I can do that?  I do have an external hard drive with way more space.  What's the best way to transfer the projects over?  Should I save them as "Cakewalk Bundle" files and then just open them on the new drive and go from there?

One thing you can do is consolidate your projects by deleting unused files. It's normal for a great many unused files to accumulate in a project as you edit it. For example, every time you split a clip the original clip stays behind even though you'll probably never reference it again.
 
The easiest way to do that is File -> Save As. That will rewrite the project to a new name (which you can change back later if you like) and will save only those files that are specifically referenced by the project file. The new project might be a fraction of the size of the original if you've spent a lot of time editing. After you've loaded the new version and verified that everything's there, you can delete the original project. 

The best way to transfer the contents of one disk to another is to simply copy everything over just as it is. That can take a long time if there’s a lot of data so may be best to start the copy then let it run overnight.

Do not delete anything off the smaller drive until you are sure the copies are good. In fact you should be backing up everything you do to a backup drive whenever you work on it. A drive can fail in seconds or develop bad sectors that make some data unreadable. Without a backup regime you have lost that data for ever unless a recovery service can get it back. And drive recovery services charge a lot of money and don’t always get back all the data either.

I don’t know if it works on Win10 but Microsoft’s free Sync Toy was excellent for doing that sort of backup job and could be scheduled to run overnight or when the computer isn’t in use. There are other backup and file copy and verification applications that can do the job as well.

As for the original disk, when a disk is full, it’s full. Which means something has to be deleted to get space back or it’s time for a new drive.

Thanks for the replies! For copying projects to another drive, which folders specifically need to be copied?

MelodicJimmy
For copying projects to another drive, which folders specifically need to be copied?

Dave just told you
 

bitflipper
The easiest way to do that is File -> Save As. That will rewrite the project to a new name (which you can change back later if you like) and will save only those files that are specifically referenced by the project file. The new project might be a fraction of the size of the original if you've spent a lot of time editing. After you've loaded the new version and verified that everything's there, you can delete the original project.