Can't boot from new drive (?)

Oct 5, 2018
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(specs below, don't know if it will help)

Recently I bought an M.2 SSD to put my Windows on. I made a bootable Windows 10 USB with Rufus and could boot into the USB no problem. While installing Windows i selected the M.2 to be the destination where Windows would be installed on.

However, it seems the boot drive is somehow linked to my old HDD. Windows was installed on the M.2, I know because its the only drive with data on it. (picture of Disk Management https://i.imgur.com/Oo3majd.png ) In Windows now I want to format all my other drives (another regular SSD and the mentioned HDD). The SSD went smoothly but the HDD has unremovable partitions. I tried deleting them in DiskPart (following this video https://www.youtube.com/watch?v=-F5DSQOAgZg ). However for me it says this https://i.imgur.com/rTFZljO.png . I found this rather weird because in the BIOS I couldn't boot out of any drive other than the HDD. In the bios it says Windows boot manager WDEZEX. (https://i.imgur.com/ex4zwHn.jpg I must have the HDD in here or else I get this message https://i.imgur.com/ofyn2Gy.jpg ) when I replace the HDD in the boot order a black screen pops up saying I need to select a proper boot device.

The HDD is the oldest part in the build, everything else had been replaced before. I have backed up all important files so fresh installing isn't a problem.
Lastly, these are all my drives currently in PC. https://i.imgur.com/6RWJxUy.jpg



So somehow I need to find out:

• why I have to select the HDD as primary boot option in BIOS

• why I can't delete the EFI partition on the HDD -OR- how I can make my M2 a boot device.



CPU: Ryzen 5 1600

RAM: Corsair Vengeance LPX DDR4-3000 16GB

Motherboard: Gigabyte B450 Aorus Pro

Storage: 1TB WD HDD, 860Evo 500GB SSD, 970Evo 250GB M.2 SSD

GPU: Asus ROG Strix RX580

Cooler: ML240L

PSU: RM550X
 
Because bootloader lives there (EFI System partition in Disk Management screenshot).

Again - bootloader lives there. If you have booted from this bootloader, you can't delete the partition.

Easy. Move bootloader to M.2 drive. Or reinstall windows with only M.2 connected.