My son’s Windows laptop did the same. Turns out there is a setting to make Windows truly shut down when selecting “shut down” from the menu, because normally it secretly sleeps or hibernates or something to have faster start-up times. There’s also the power another device via USB option that you may have to disable in BIOS / EFI settings.

So I downloaded slackware on dozens of disks.

This is no joke. When I downloaded Slackware in '95 or '96, it was over 100 3.5" floppies of 1.44 MB each. And there were still more available, those were just the ones I thought I’d need. And before you could even begin installing, each of those had to be downloaded, written and verified because floppies were not terribly reliable.