won't boot into Windows 10 when USB flash drive is inserted

atexit8 said:

...I no longer can use F12 to choose the Boot Menu.

Yes, this is the case if you have Fast Start up turned on in Windows 10 (by default it is turned on). This is a form of hibernation, so the boot choice bios screen is skipped on powering up. You may also find that with Fast Start up turned on you don't get the option to enter the bios setup either.

Either turn off Fast Start up, or use Restart (rather than Shut down). A Restart doesn't use Fast Start up so you will get a chance to enter the bios setup or to use F12 to choose the boot device.

Before installing Windows 10, I had changed the boot order in BIOS to USB HDD, USB CD ROM, ATAPI CD ROM, HDD.
What happens now instead is that the laptop will not boot into Windows 10 when a USB hard drive or a USB thumb drive is inserted into any of the 4 USB ports.
The USB drives contain only data files and are not bootable.

Can someone explain why this is happening and how do I fix it?

You set USB as the first boot device in the boot priority. If it finds a USB drive connected it doesn't skip the USB if it's not bootable, it will try to boot from it and will complain that the OS is missing if it is not a bootable USB. Only if all the other boot devices are not connected will it boot from the HDD as you have set that as last in the boot priority.

Fix it by putting the HDD ahead of the USB drives in the boot priority. You can leave the CD ROM ahead of the HDD if you like, you're less likely to leave a disk in the DVD drive so that shouldn't be a problem.

Isn't that F12 a BIOS function to choose a BOOT drive to BOOT from ? It looks to me that USB is chosen to be default BOOT device in BIOS. If that's the case, than BIOS should be set to BOOT from HDD.

atexit8 said:

I disabled the Fast Start in Windows 10, but I still don't see the Boot Menu.

I still can get to BIOS using F1.

These are Lenovo's instructions for getting to the bios or boot menu. They don't appear to be any different for F1 or F12. Sometimes you have to be very quick, so try pressing F12 repeatedly starting before you see the Lenovo logo.

https://support.lenovo.com/gb/en/solutions/ht500222

But apparently on a Lenovo 510-15 there's a small hidden reset button you can only press with a thin object like a pin or a paperclip. See this Lenovo forum post.

https://forums.lenovo.com/t5/Lenovo-...5/td-p/3559438

Please enlighten me, @Bree. If you go into Settings → Update & Security → Recovery → Advanced Startup, can't you force the PC to boot from alternate media even if Fast Startup is turned on? That's what I do routinely on my PCs when I want to boot into my recovery partition or from an USB flash drive. Am I missing something? Thanks in advance for your upcoming response.
Best wishes,
--Ed--

EdTittel said:

Please enlighten me, @Bree. If you go into Settings → Update & Security → Recovery → Advanced Startup, can't you force the PC to boot from alternate media even if Fast Startup is turned on? That's what I do routinely...

Yes @EdTittel, that's Option One here...
Boot from USB Drive on Windows 10 PC

Although, on my PCs I only have four of the five option shown in Brink's screenshot - I'm missing the 'Use a device' one. That may well be because it's a UEFI-only option. Mine are Legacy bios systems, as is the OP's system...

atexit8 said:

... used Thinkpad SL510... It is an older laptop so no UEFI.

But even if they could boot from USB that way, it would have been a workaround, not the fix. The OP's problem (now solved) was that their bios was set up in such a way that it bypassed offering the chance to use F12 for a boot choice menu.

@Bree: thanks for the explanation. I knew there had to be a good reason. As I mentioned last week in the context of a different boot/system drive scenario, I'm now in the situation where I have only UEFI systems, so I don't always stop to consider things from the traditional BIOS perspective. I didn't realize that was involved here, too, but it makes sense.
Thanks again,
--Ed--