Can't Read/Write to DVD

Unfortunately, not even Mac OS can add more content to an RW disc (unless you erase the current contents first). There used to be more of a call for software that could do multi-session writing/burning, but I don't know if there is any app out there today (for a Mac). I used to use it with Toast v. 5 (if I remember correctly). It came in handy when you wanted to add more files to a backup CD RW (before the age of external drives, USB, FW, etc). I still have a box full of (empty) CD and DVD RW discs.

If you are trying to add more content, you cannot do that without special software unlessyou erase the disk first.

It would also help to actually post important info such as your device, OS version, and exactly what it is you are trying to accomplish and how you are trying to do it.

I know Roxio Toast used to do that some years back - do not know if the latest version still offers that option. May want to ask them before paying money for it. Any other way, the CD or DVD needs to be erased and then you can re-record/write to it again.

The way you write to a disc is very important. You do not drag one file after another to the disc itself.

Mac OS software can do this for you, but you have to proceed just the right way. You usually accumulate the stuff you want to burn into a burn folder, then tell it to burn all that stuff to disk. This article describes the process;

Burn CDs and DVDs on Mac - Apple Support

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It is said that Steve Jobs demoed that on stage once, and the audience applauded.

He told them, "Don't applaud, that is the way it SHOULD work!"