"No boot medium was found" but FAQ answer to that is unrelated.

[jdgalt]

Q: When I start my VM, I get an error stating that no boot medium was found. How do I solve this?
A: Please read more about Virtual Machines so you know what VirtualBox actually does. Basically it comes to that it 'simulates' a computer with hardware and everything. The Guest does not know about the Host or it's hardware. So you have to install a seperate Operating System in it before you can really use VirtualBox.
Also make sure that if you want to boot from a CD/DVD or ISO, that it actually is bootable.

The above is from the FAQ, and the question exactly matches my problem but the answer has no relevance to me at all.

My hardware is an AMD Athlon 5150 (64 bit) with 64GB of RAM, but its only peripherals are its VGA monitor, 2TB SATA hard drive, and a USB 2 controller, into which are plugged my keyboard, mouse, and the DVD-RW drive from which I need to boot. The drive has in it an original Windows 98 boot CD. Host operating system is Ubuntu 17.04 (64 bit).

VirtualBox allows me to create, and start, a virtual machine named "Windows 98", and VB's console window shows that the existence of the optical drive is recognized:
"IDE Secondary Master: [Optical Drive] Host Drive ASUS SDRW-08D2S-U (sr0)"

And still it refuses to boot Windows 98, but pretends that no CD is in the drive. How do I make it work?

(Yes, I did read the "minimum requirements" post, but I don't think I've gotten far enough that I could meaningfully comply.)

[socratis]

And still it refuses to boot Windows 98, but pretends that no CD is in the drive.

I'm not sure it pretends that there is no CD in the drive, as much as that the CD is not actually bootable. Do you have any proof that the CD that you have in your host drive is bootable?

a USB 2 controller, into which are plugged my keyboard, mouse, and the DVD-RW drive from which I need to boot.

Your CD is not an internal one, it's an external one (it's not ATAPI, it's USB). VirtualBox will not boot from such an arrangement. Your best option is to create an ISO file from that CD. imgBurn is an application that can help you with that. Then, instead of pointing VirtualBox to the CD, point it to the ISO. It's a lot faster.

the existence of the optical drive is recognized: "IDE Secondary Master: [Optical Drive] Host Drive ASUS SDRW-08D2S-U (sr0)"

Are you referring to the VirtualMachine settings » Storage dialog? I would like to see a screenshot of that.

(Yes, I did read the "minimum requirements" post, but I don't think I've gotten far enough that I could meaningfully comply.)

Actually that's a mistaken assumption. For example I have no idea what your host is or your VirtualBox version is. For the first, it would be really helpful to suggest a method to create the ISO. The one I suggested applies to Windows hosts only. If not on Windows, you've got to figure it out on your own. It wouldn't have killed you to add two lines so that we all know where we stand, would it?

[The Raven]

Your CD is not an internal one, it's an external one (it's not ATAPI, it's USB).
VirtualBox will not boot from such an arrangement.

I've never had any problem booting a guest from a USB attached CD drive.
My test machine has no internal CD drive at all, so I always have to install
guest OS's using an external USB drive, albeit attached directly to a host USB port.

I'm assuming when the OP says USB2 controller it's a hub, either externally
powered or from the USB port it's attached to. I'd never tried using a CD drive
attached to hub, so I tried that (not externally powered) and that worked as well.
Booted two different OS discs and a Standalone Disk Dump/Restore application.

The only oddity I do remember was with Windows 7 (I think) where it would boot
from a USB attached CD but fail during the install. It only happened if you hadn't
ticked the passthough option on the attributes section of the CD in the storage settings.

I have no idea what your host is

OP stated

Host operating system is Ubuntu 17.04 (64 bit).

[socratis]

I've never had any problem booting a guest from a USB attached CD drive.

So the external USB CD appears in the drop-down list of the CD storage settings as a Host device? And you can use the passthrough option? I don't have an external CD drive, but I'm expecting to have one within the week, so that would be welcomed news.

It was so that only the internal CDs were present as Host devices. No USB ones, only the ATAPI ones. I didn't even realize that this option was available/working...

[BillG]

Yes, it works fine for me too. HP USB External DVD drive plugged into a USB 2.0 port. (I had never tried it before, but it works).

[mpack]

Yes, it works fine for me too. HP USB External DVD drive plugged into a USB 2.0 port. (I had never tried it before, but it works).

Did this external DVD come with its own drivers, or does it use native host mass storage drivers?

It's my understanding that if the USB device relies on the host OS's USB mass storage drivers then VirtualBox can't tell it apart from other mass storage (e.g. thumb drives) and you can't use it like a CD/DVD. We've had many topics that boiled down to this over the years. Mostly on OS X hosts, but I think that's mostly because they were the first to abandon CD drives in portable devices.

[mpack]

Regarding the OPs problem. If you get the "No bootable medium found" error, then the fix is to provide bootable media. If your CD is connected to the VM and you get that error then the CD is not bootable.

This used to be a common problem reported on these forums. There was a Win98 sampler/demo CD that was freely available, on covers of magazines etc. It had the Windows 98 logo in big letters and people finding them years later simply assumed that this was an installer. It wasn't. It was just a demo intended to be run under Win95. It wasn't bootable.

If you have a Win98SE Setup CD then it will say all of those things on the cover. If it just says "Windows 98" - no mention of setup - then it isn't a setup cd.

[The Raven]

Did this external DVD come with its own drivers, or does it use native host mass storage drivers?

My hosts are Linux - Debian 8.
On those, the external USB drive is treated as CD drive rather than a mass storage device (thumb drive).

As a test on my main machine (which has a proper internal CD drive), I attached both my USB CD drive
and a bootable USB stick.
When I go into the "Choose Virtual Optical DIsk file" option of the Optical Drive settings for a guest, both
the CD devices are listed as follows :
Host Drive - <manufacturer details> - (sr0) <- this is the internal drive
Host Drive - <manufacturer details> - (sr1) <- this is the USB attached

As you say, the USB stick (which is also bootable) is not shown as a device.

[socratis]

My external CD arrived today and lo and behold, it appears as a "Host Drive 'Apple Super Drive'". Passthrough is available, but I've got nothing at the moment to test that feature. Booting from the external USB CD drive works just fine.

• Host: OSX 10.11.6
• VirtualBox: 5.1.25 r117130 (Qt5.6.2)
• Apple SuperDrive:
  • Firmware Revision: 2,03
  • Interconnect: USB
  • Burn Support: Yes (Generic Drive Support)
  • Reads DVD: Yes
  • CD-Write: -R, -RW
  • DVD-Write: -R, -RW
  • Write Strategies: CD-TAO, CD-SAO, CD-Raw, DVD-DAO

So, something seems to have changed in VirtualBox and now the USB CD drives are presented as equals to the built-in ATAPI ones. Excellent news !

[mpack]

So, something seems to have changed in VirtualBox and now the USB CD drives are presented as equals to the built-in ATAPI ones. Excellent news !

Perhaps, or perhaps it's the drive which is different from past examples, e.g. by not being treated as generic mass storage.