How can I add new content in iDVD to a DVD-RW disc which has ample remaining free space? After preparing the new video for burning and clicking on Burn to iDVD, I get a window saying the disc's already recorded and that I can either Erase or Eject.

You can't. Any movie disk has to be "closed" in order for it to work. Closed means nothing else can be written to the disk regardless of how much space is unused.

Data disks can be written to until full as long as you remember to write each group of data as a session.

Since it's a rewritable disk, what you'd have to do is erase the disk. Then go back into iMovie so you create a project where both the previous movie and the new one can be sent in to iDVD as a single project. Then both movies can be written to the disk, each with their own start button.

There are, but not with a DVD written as a movie disk. It must be closed when completed, or it doesn't work.

Apple's built in Burn utility also automatically closes any data CD, DVD or Blu-ray disk you burn. Doesn't matter how much space is unused, you can't use it. You'd have to use a more advanced disk creation app, such as Toast Titanium. I then have the option of choosing to write the data as a session:

I can keep doing this until the disk is full. If I've written five sessions to the disk, when I put it in the drive, five CD/DVD icons will appear on the desktop since the OS will treat each session as if they are separate physical disks. At any point you choose Write Disk when writing a group of data, that means you're closing the disk, and again can't add anything after that. So if I had written two sessions, and the third was Write Disk, it's over. I can't put anything else on that disk.

Another item to not is that DVD-RW disks are notoriously unreliable when it comes to video DVDs. Good DVD disks are very inexpensive these days and burning your edited video DVD project in iDVD to a new disk is pretty inexpensive.

Follow this workflow to help assure the best qualty video DVD:

Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.

To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.

Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.

Another item to not is that DVD-RW disks are notoriously unreliable when it comes to video DVDs. Good DVD disks are very inexpensive these days and burning your edited video DVD project in iDVD to a new disk is pretty inexpensive.

Very good point, Old Toad! Many set top players have trouble reading the much darker dye layers of rewritable media. Much better to use write-once disks for movies.