It's unfortunate that Windows 10 beat Slackware 14.2 to market.

If removing the CMOS battery doesn't work, you may have to physically remove the boot drive and reformat it.

A decade ago, I gave up trying to dual-boot because Microsoft would sabotage the MBR. I figured "Since Microsoft doesn't want me to dual-boot, I will run only Linux". Now I build my own computers and run Linux exclusively.
Ed

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There do exist vendors that sell computers with Linux pre-loaded, so there should always an out, if you really want to use Linux. Right away, only System 76 comes to mind, but GNU also keeps a list of vendors at https://www.gnu.org/links/companies.html.

Otherwise, I'll trust that at least the major vendors will find a way to get their Linux onto a Secure Boot computer. I'm not sure what mandatory Secure Boot would mean if I had to roll my own kernel, though.

The very last non-Secure-Boot computers are rather quick and will hold me over until someone in the open source community thinks of a fix or workaround that pleases everyone. Have faith.

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It is fortunate that Slackware does not compete in the "shiny new version" market race, and its users remain stable, happy and mostly unaffected across version releases, no matter how fast or slow they come.

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One more thing: Will brand-new computers preloaded with Windows 7 work if Windows 7 is removed and Slackware loaded in its place? I ask only because even to this day, Dell and perhaps other vendors offer brand-new computers with Windows 7 preloaded. It could be that Linux users end up in the same boat with those of us who were trying to avoid Windows 8 at all costs.

FWIW, I do have a month-old Dell here, preloaded with Windows 7 Professional, and Dell configured it to boot in Legacy mode, with a choice of UEFI in the BIOS. I'd only assume that it works with Slackware. It's a production PC, so that assumption will not be tested until the PC is retired some years from now.

I upgraded to 10 on my HP laptop. It borked by boot loader. Reinstall and seems fine now.

You probaly had a UEFI system, with originally legacy bios boot allowed, the Windows 10 upgrade probably switched it to UEFI secureboot, so legacy bios boots aren't allowed until it is removed.

Secureboot needs to be disabled inside Windows 10, it won't show up as a changeable setting in your UEFI menus, because that would defeat the purpose of secureboot.

So boot into Windows 10, from the new start menu choose Settings, then go to Update & Security, then Recovery, then Advanced Startup -- restart now button

After this I'm less sure, as I have my test Windows 10 on a machine with legacy bios

but on Windows 8.1 this would bring up a screen, which you would choose Troubleshoot, then you would choose Advance options, then UEFI firmware settings, then restart to change.

It will reboot into UEFI settings in a way that they can be altered and hopefully turn off secureboot, and maybe also turn on legacy bios boot as well. Just be warned if you want to use bitlocker for full disk encryption, secureboot must be on.

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Yes, just stick the slackware dvd into the drive, boot to that and then let it reformat your drive. That said some dell computers do not work with all GNU/Linux distros, I used to have one that would work with nothing, but opensuse and gentoo. I read recently here about someone that had one that would only work with slackware... Legacy boot will be a lot less of a pain than uefi and I really can't see any reason to use uefi unless you want to dual boot windows.

No Operating System should have access to the CMOS/UEFI interface to change any settings. That is dangerous activity.

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Er, what? What does efibootmgr do on Linux?

The Slackware installer itself offers to add an entry to the UEFI boot menu if you run it in UEFI mode.

Changing settings is one thing, but accessing it in read-only mode is another to appropriately configure things within the OS and bootloader(s). Plus, you should be told or given the option to know what it has changed, especially if it changes a setting that locks out other operating systems, and more importantly, it shouldn't be accessing the UEFI in write-mode at all, period.

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Only if you respect your users. Thus you see it in FREE software, and you see the inverse in exploit-ware OSs.

But of course, that was almost certainly the intent. These things are not just poor implementations or forgotten side effects, they are well considered "features" from the malign entity we know as M$ corporate.

...The more things remain the same ...

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The sad part is Microsoft is the only entity here getting truly hurt. The more they try to hurt BSD, Linux, Illumos, Solaris, etc. the more they end up hurting themselves in the end.

Maybe you should have read the UEFI specification before posting this statement as this ability (be it by the OS or any compliant software) is at the core of the spec. Additionally, since version 2.5 the way an OS can update the firmware through the ESRT (EFI System Resource Table and component firmware updates) is specified and the new support to expose the EFI System Resource Tables in sysfs will be implemented in Linux 4.2, see this pull request.

Now, if you really meant that UEFI is intrinsically evil, just state that. Good lock to have it changed or deleted, then.

If you have Slackware on your hard disk (not on USB) you can reconfigure your boot loader to use both Windows and Slackware by some third party boot managers through Windows.

EasyBCD is a good freeware for this purpose. I don't know anything about its compatibility with Windows 10 but it works excellent on Windows 7 and on both CMOS/UEFI systems.

Give it a try before touching your system Bios. You can use this guide about making a multiple boot system using EasyBCD.

PS I think you should pick a better title for your thread.

Sorry for my english

Well.

I've got Win7 in VirtualBox and it works as well as anything from Microsoft ever does and it can't hurt anything else. I don't consider an "upgrade" to Win10 to be of any real use unless Win10 will run without any problems in VirtualBox (which may be unlikely for... a year or two?).

Win10, methinks, is like those little dancing dogs in the circus: the question is not how well but rather why at all.