Switched to legacy boot to backup my windows8.1 part before going to win10. Now, after backup and reboot, can't get into hd windows8 or the bios the normal way (f2) or any key for that matter, stuck in network boot till i insert cd or usb. Only solutions to reset legacy to uefi I haven't tried is to format and install windows 8 or 10, a cd version won't work. Considering hd format to mbr, reinstalling something and hoping to get to fuller list of advanced options and thus the bios again, just to switch back to gpt... sounds crazy. Also considering cmos wipe if I could figure out how.
Ok so when it's trying to boot in legacy mode it will see the hard disk as unformatted. Therefore it won't boot Windows 8 or 10. You need to get the bios back into booting uefi.
Resetting the bios should do that anyway, but there is no reason why you shouldn't be able to get into the bios to make that change. Try doing this with the hard disk unplugged so you don't risk trashing it.
Testimonial: "Laptop is a Samsung Series 5 (NP520U4C) with phoenix bios (p02ral i think) - 'f2 for bios'. Doesn't seem to respond to any keys. Without legacy os in cd or usb, it attempts pxe net booting twice before shutting down, then booting again by itself. Oddly, with drive removed, the 'f4 reset' seems to head off to outer space for a while before finally rebooting. I can't seem to find a bios flasher either, tried one from http://sbuservice.samsungmobile.com/upload/BIOSUpdateItem/, using older UBCD but xp needs xmlLite.dll to actually run it, and then ultimately tells me my fully changed laptop is at less than 30%, so no go anyway (if I could boot from the intact gpt disk, well then, grr, but then thats the problem...) I'm gonna continue trying to clear the cmos for a few more hours, but ultimately will try to put a new mbr token windows install (ie all partitions are forfeit) in hopes that shift-restart will get me back into the bios menu, maybe on the first install reboot. Would be nice if I could actually alloc the mbr part in uncharted territory, and then use something like testdisk to get them all back. Thanks for your help."
I ultimately got the bios back, without sending the machine in to samsung or whereever. Should mention that draining the cmos was the right solution, but in practice, sent me down multiple path that would make it cheaper to have sent it in. Key points, 1) samsung has a non-removable battery. You must push a pin thru a small hole on the bottom, clearly marked for those who know what to do, to turn off the laptop battery. Be sure to test turning it on to make sure its off and to drain whatever charge is left in the capacitors. Plugging it in will turn it back on later 2) take the thing apart again, unplug the cmos battery and let it sit for a while. This resets the bios to default settings, returning my efi boot in this case.
Kind of a poor design to rely on a functioning windows to get to such a key feature, but such is the way of things. Thanks to the millions of resources I had to pore thru to solve this. Nod to samsung India for being supremely pleasant and unhelpful.
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