When you insert recordable media—CD-R, DVD-R, etc.—into your Mac’s SuperDrive, the Finder will pop-up a dialog asking what you’d like to do with it. If you click Ignore, perhaps because you intend to burn the disc later in Toast, the Finder won’t mount the disc. Eventually (if you’re like me) you’ll either forget about it entirely, or remember you inserted a disc, but can’t remember if it was a CD-R, or a single- or dual-layer DVD-R.
You can find out, of course, by hitting the Eject key on the keyboard. Short of that drastic measure, here are a number of alternative ways to answer the question without opening the drive tray. First, you can use Disk Utility, in Applications -> Utilities. Select your optical drive in the sources list, and then look at the bottom of the Disk Utility window. Among the other items shown is Media Type, and that answers the question—it will read CD-R, DVD-R, DVD-ROM, etc. based on what’s inserted in the drive.
You can find the same information in System Profiler. Hold down the Option key, click the Apple menu, select and select System Profiler. Next, click on Disc Burning in the Contents column, click on your optical drive in the Burning Device section of the window, and then look in the Media section below—you’ll see not only the media type, but also whether or not it’s blank, erasable, overwritable, appendable, and the disc’s available write speeds. This is the most-detailed information about the inserted media that I was able to find.
Finally, if you’re Terminal-inclined, launch Terminal and type drutil status:
$ drutil status Vendor Product Rev SONY DVD RW DRU-840A SS00 Type: DVD+R DL Name: /dev/disk4 Write Speeds: 4x, 6x, 8x Overwritable: 927:30:74 blocks: 4173824 / 8.55GB / 7.96GiB Space Free: 927:30:74 blocks: 4173824 / 8.55GB / 7.96GiB Space Used: 00:00:00 blocks: 0 / 0.00MB / 0.00MiB Layer 0: blocks: 2086912 / 4.27GB / 3.98GiB Writability: appendable, blank, overwritable Book Type: DVD+R DL (v1) Media ID: RITEK S04
The Type entry displays the type of media (a dual-layer DVD+R in this case), and the Writability entry describes the attributes of that media.
Thanks to Mac OS X Hints reader Daniel Fazekas for the Terminal method; I noticed the information in Disk Utility and System Profiler while trying to help a reader with a disc burning question.