Hi I just got another PC with UEFI things and so not to mention much more powerful hardware.
And decided to put my old SSD with Slackware into it, besides other partitions.
Now I wonder what is the best way to get Slackware back running without messing too much with existing things.

Maybe there is way I can use windows 10 bootloader somehow?
Other simple ideas also welcome.

Well, you can have multiple ESP (EFI) partition in a computer. One in each hard disk.

Assuming that your Slackware bootloader from the ESP of second hard driver is on the path: /EFI/Slackware/grubx64.efi (but similar is also for ELILO), I for one I will install rEFInd on /EFI/Boot (on the same ESP partition) and it will auto-discover both the operating systems, giving you a nice graphical boot menu for them.

After all, the rEFInd is first of all a Boot Manager, not a Boot Loader, even it have this ability too.

Of course, you should chose first from UEFI firmware which hard drive boots first in UEFI mode.

Bonus points: this way you do not touch the Windows and its hard disk at all.

Can you give more details? Like some link to tutorial?
Keep in mind that at this point my Slackware is not bootable and not accessible.

I suggest you download a LiveSlak ISO and install it in an USB drive via Rufus, for example. OR, even better, by using Ventoy.

https://download.liveslak.org/

https://docs.slackware.com/slackware:liveslak

This way, you can boot a Linux OS and inspect your old Slackware as you like.

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I installed Ventoy into my USB drive and placed Slackware iso into it.
Got though Slackware selection screen but got stuck during boot.
It really works that way?

In theory, yes. In practice, of course also depends on the hardware.

Anyway, there's what Mr. Hameleers says about creating and booting an USB dongle with LiveSlak without Slackware.

https://alien.slackbook.org/blog/how...ing-slackware/

BTW, better to read also the comments on this blog post.

Also, see there:

https://www.linuxquestions.org/quest...nu-4175687644/

Is "Secure Boot" enabled in your UEFI settings? Slackware won't boot with it enabled. Windows 10 will still work if you disable it.

For dual booting, you have two choices: rEFInd https://www.rodsbooks.com/refind/ or grub2.

Grub2 is included in Slackware by default, but a bit more finicky to set up. rEFInd can, more or less, just be dropped into place.

And the biggest question is: can be disabled the Secure Boot?

A brand new computer today may be Secure Boot only, from what I seen. Specially those "designed for Microsoft Windows 11"

Typically, the UEFI BIOSes with no CSM has also no ability to disable the Secure Boot. And like was said, Slackware has no ability to boot under Secure Boot.

So, if you do not find CSM functions in BIOS, and considering also that our BDFL probably will "consider" another 15-20 years either to sign or not his bootloaders and kernels, the best way probably is either to go Ubuntu, either to enjoy your Windows 11. After all, you payed for it.

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I believe I disabled the secure boot. And it booted and I saw penguins and so, but the kernel got stuck. Too bad I forgot to take a pic with my phone.
I have that option on my ~5-year-old motherboard.
I will try Rufus when I have more time. It seems less complex. For sure I will take pictures then.

Some Slackware installers still drop one to a prompt where boot options can be designated including kernel and root device selection, allowing one to boot an existing system with the available kernels on the installer. I have no idea why this isn't default action anymore along with examples and instructions but at least sometimes this still works.

However once any kernel from the same Arch boots to a full bash runlevel, chroot is possible and a great tool for fixing a system with any manner of kernel panic among many other rescue missions.

For simple reference

Once effectively in the system at that root location to be fixed, one can install Grub2, rEFInd. or simply run liloconfig. elilioconfig, efibootmgr or manually alter a boot menu to edit or add a different kernel or initrd, or convert an MBR partition to GPT, whatever you think you need to restore a normal, working boot process.

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If you can boot either Ubuntu or Fedora on such a machine, you have access to shim, because they both use it. And shim allows you to set up a Machine Owner Key (MOK) to sign any bootloader you want to use. Generally you do this during the installation. So once you have your key pair, you can sign the slackware version of GRUB or elilo and the kernel too if you need to.

There's also a slackdoc telling you how to do it without shim by using efitools to create your own platform key and key exchange key. There's always a way around these things.

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Minutes before I retried with Rufus and got quite same results.
But this time I made picture with phone:
https://drive.google.com/file/d/1jM7...U-qe6TYQm/view
Any ideas how to resolve this issue?

Try another USB port and/or try another USB dongle.

BTW, try to connect the dongle to the case's back USB ports, not on the front ones, which are connected internally via a cable.

Apparently the troubles are at very low levels, as in failing to properly detect the device associated to your storage on USB dongle.

I have seen errors like this with faulty USB dongles or ports.

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I tired another port on back already with pretty much same results.
I think I will try to get some USB 3.0 dongle few days later as this one is really old one.

Try another port in the back, and ensure that it's an USB 2.0 port.

That's a good idea.