Latitude E6420 shows DVD-RW as read-only

Any firmware is only going to be able to use a CD-R/RW/+R/+RW as a boot medium if it's been properly written -- there's no such things a a writeable CD/DVD at the firmware level.

You will need CD/DVD writing software for whatever Linux distribution you have.

I apologize for not giving adequate information.

Here are the relevant sections of lshw

humphrey@Gummitch:~$ sudo lshw
[sudo] password for humphrey: 
gummitch
    description: Laptop
    product: Latitude E6420
    vendor: Dell Inc.
    version: 01
    serial: 1CB23R1
      capabilities: smbios-2.6 dmi-2.6 smp vsyscall32
    configuration: boot=normal chassis=laptop uuid=4c4c4544-0043-4210-8032-b1c04f335231
[...]
        *-raid
             description: RAID bus controller
             product: 82801 Mobile SATA Controller [RAID mode]
             vendor: Intel Corporation
             physical id: 1f.2
             bus info: pci@0000:00:1f.2
             logical name: scsi0
             logical name: scsi1
             version: 04
                         clock: 66MHz
             capabilities: raid msi pm bus_master cap_list emulated
             configuration: driver=ahci latency=0
             resources: irq:27 ioport:40d0(size=8) ioport:40c0(size=4) ioport:40b0(size=8) ioport:40a0(size=4) ioport:4060(size=32) memory:e2e40000-e2e407ff
[...]
           *-disk
[...]
           *-cdrom
                description: DVD-RAM writer
                product: DVD+-RW AD-7930H
                vendor: Optiarc
                physical id: 1
                bus info: scsi@1:0.0.0
                logical name: /dev/cdrom
                logical name: /dev/sr0
                version: 100A
                capabilities: removable audio cd-r cd-rw dvd dvd-r dvd-ram
                configuration: ansiversion=5 status=nodisc
[...]

I assure you that there is a disk in the device.

Why is the system not recognizing the disk?

If the disc is blank, it has no filesystem on it to mount -- this is completely normal. Or is there a filesystem on the disc?  If there is, and the drive doesn't read it, it's relatively old if it came with that system -- and may be defective.

@ejn63​ Any suggestions how I can test the drive for being defective?

If the disc has a UX-readable filesystem on it but the system won't mount it, the drive is faulty.

@ejn63​ It was a plain music CD (purchased in a shop)

I now have a Matshita DVD+/-RW which works.  Initially it failed to read a music CD and the advice I got was that the device might need a firmware update .  However, thatt seems to me a quite unrealistic idea for a basic device like a CD/DVD read/writer.  Would you agree?

@ejn63​ okay, that makes sense.  Thank you.