Reconnected HDD won't boot but it should

Details: ASUS M11AA desktop UEFI

Running Win10 on an HDD, I disconnected the sata cable to it and tested out an SSD on a diff sata cable. Installed 10 on that just for fun.

Took the SSD out, reconnected the HDD sata cable and it wont boot. The HDD is seen in the bios but in the boot menu it is not seen. I see items such as UEFI: Sandisk(my acronis usb stick), UEFI: P3 DVDRW, UEFI: Kingston(my win10 install usb stick), Windows Boot Manager but not UEFI: P2: Toshiba which im pretty sure is what my HDD was.

Windows Boot Manager goes to UEFI setup.

Any clues before I try Win10 repair from the Kingston stick?

Hello FrostyWinnipeg Welcome to the Ten Forums!

If you are keeping the two flash drives plugged in try unplugging both to start or the San Disk in order to boot live from the Kingston. Instead of the Upgrade to Repair or slapping on another clean install you will first want to try a few things out like the Repair Startup tool and going to the Command prompt option run the DiskPart tool in order to have that list the drives with the List Disk command. Once you ID the correct drive and select that type in "active" to see the primary made bootable.

The other thing you may end up doing is double checking the data cable you unplugged to be sure that is all the way in at both ended and didn't jar loose at the board end by chance. You have need a can of compressed air to blast out any dust or debris that could have got lodged in as well.

When putting the SSD did you make any change in setting at the time? That would explain why you are seeing immediate results when just going in now to make that change. At least you had a quick fix seen there!

I've had a few incidents? at times when dual booting OSs and changing boot orders as well as rearranging cables between drives where one time nothing would start and then next time you suddenly saw things running normally again. On one occasion I even had the host drive's cable plugged into one of the storage drives until deciding to mark numbers on each one to show which port it was going to. Your change of setting apparently made the bios search for Legacy not UEFI.