@ shanper.
Thinks of a USB-Key as a tool. They come in handy when you want to try something without modifying your working operating system. They are indispensable when you are attempting something which can turn your computer into a paper weight. Among those somethings are (1) reformatting the partition on which your operating system resides and (2) changing your boot-loader. Being able to boot an operating system from a USB-Key after you've done something to the hard drive which rendered all operating systems on it inoperable can be thought of as a safety net. But it also provides far more flexibility with what you can do with the computer itself.
There are a couple devs on the Forum who repair computers. Puppy-on-a-stick is an essential part of their tool-kit.
That it can be done doesn't mean it should be done. I've read reports of cats falling out of windows on the 12th floor and landing safely on their feet. Forcing cats out of a high level windows is still not recommended.
A USB-Key doesn't have to be large. Puppys don't take up much room. I actually find having a couple GOOD QUALITY keys such as the 8GB SanDisk --on ebay for $4.85 each-- provides greater flexibility than one large Key.
But you may have other options. Tell us more about what you now have. How large is sda1 from which Ubuntu boots? Is it an encrypted partition? Do you know how it is formatted: i.e., is it Linux Ext4 64bit? If you don't know the answer to that, telling us which version of Ubuntu is installed, created it, would help.
Puppys are designed to co-exist with any other operating system. So you may be able to do the following:
1) Install grub2config, https://www.forum.puppylinux.com/viewto ... 703#p29703 in your Puppy on sda4
5) run grub2config
grub2config will over-write your current boot-loader but should enable you to boot into either Ubuntu and Puppy now on sda4. [Should is an iffy word; why I recommend having a backup plan].
6) Having booted into that Puppy you can then manually delete --not reformat the drive-- everything from sda1 NOT relating to your new boot-loader. The last part is tricky. Ask for specific advice as to what not to delete. Best provide a screenshot of what your sda1 looks like just before you start deleting.
6a) Probably safer. Since you're using Puppy to do the deletion and you've installed grub2config into it, after the deletion --before you shut down-- run grub2config again.
7) Once you can boot into Puppy on sda4, you can add any Puppy on sda1, including another instance of BionicPup32 19.03. Puppys only need there own folder. Just give that folder a unique name --e.g. bionic2-- so that on boot-up it can be distinguished from the other.
Run grub2config again so that it will have a menu entry for the newly added Puppy.
Edit grub2config's title for it to something like:
menuentry 'Bionic on sda1 (sda1/bionic2)'{
grub2config creates a TEXT file named grub.cfg. You could manually edit any written by grub2config rather than running grub2config again. Editing it is safer --see my first paragraph-- if you know how. Safer because you're only changing a text file rather than writing several files and executables. Ask for specifics. But my general advice is edit twice. The first time just add another stanza. Don't delete or change one you know works. Boot up selecting the new stanza. It successful, you can edit again to change/delete the old.
An example of a full grub.cfg stanza might look something like this:
menuentry "Puppy bionicpup32 8.0" {
search --no-floppy --fs-uuid --set 39ff128d-806e-45a0-9c0c-4013859ec05d
echo "Loading vmlinuz"
linux /bionic32/vmlinuz pmedia=ataflash pdrv=39ff128d-806e-45a0-9c0c-4013859ec05d psubdir=/bionic32 pfix=fsck,fsckp
echo "Loading initrd.gz"
initrd /bionic32/initrd.gz
}
Some of the above isn't necessary. And there are other arguments which could be included. grub2config wrote the uuid and pdrv specifics.