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Why Can't dd Create a Bootable ISO from an Ubuntu VM?

cn flag

I have an Ubuntu 21.04 guest in VirtualBox, on either Linux or Windows hosts. I can run dd (from that guest or from some other boot in the VM) to create an image of the guest, or to clone it to an external drive.

If dd can do those things, why can it not create a bootable ISO of the Ubuntu guest? Or if it can, could someone please describe the procedure?

Nmath avatar
ng flag
It's not clear what you are asking or what problem you are experiencing. `dd` makes a bit-for-bit copy from a source to a destination. They are identical. Perhaps you should rephrase your question to ask for a solution for whatever problem you are experiencing.
lu flag
If you make the copy of the whole device from inside the virtual machine, it should be bootable. from outside, probably not. look for tools for that virtual machine system and see if there is a tool that can make a raw image file from the way it stores images in a file for compactness or efficiency.
cn flag
@Nmath - thanks for the quick reply. Could you help me understand what is unclear? I want to know how to use dd to create a bootable ISO of the Linux guest, if that is possible. If it is not possible, I would like to understand why not. I guess maybe you are saying that the contents of a bootable ISO cannot be identical to the contents of a VM? But I think the output of *dd* can be run through e.g., gzip to produce a result that is not bit-for-bit identical to the source. So a question would be whether there is some similar possibility, so as to create a bootable ISO.
cn flag
@Skaperen - ditto re quick reply. Creating a raw image will be slow and bulky, no? But if that's the only way, then OK. I was hoping for a direct-to-ISO dd command that my searching has not yet turned up. A *bootable* ISO, that is.
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