I am confused what you are trying to convert?

The SSD?

What is the purpose of the Easy2Boot external? What is that?

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@littlepyro  I'm a bit confused here too.  Are you just trying to perform a clean Windows 10 installation?  If so, you don't need to set the disk up as GPT in advance.  Just boot your Windows 10 installation media in UEFI mode, run the diskpart "clean" command if you want to (which you can do by just pressing Shift+F10 to open Command Prompt or maybe Shift+Fn+F10 depending on your keyboard settings -- no need to open Troubleshooting), then proceed through Windows Setup.  If you cleaned the disk, the step asking you where to install Windows should show only one entry for that disk listing it as "Unallocated space".  In that case, select that disk and Windows Setup will handle setting it up as GPT and creating the proper partitions for a UEFI installation.  And even for people who don't know how to use Command Prompt and diskpart, they can just delete all existing partitions at the step i just described UNTIL their target disk shows as completely unallocated space and then proceed.

I'm not sure why you're having so much trouble setting your disk up as GPT manually, since that should definitely work, but if you're just trying to get Windows installed, you don't have to deal with that. You also don't need any special utilities like this Easy2Boot tool you mentioned because Windows 10 installation media natively supports being booted in UEFI mode as long as you prep it properly.  The easiest way to ensure that is to use Microsoft's Media Creation Tool, which will handle downloading Windows 10 and creating a bootable flash drive that will support both Legacy BIOS and UEFI booting.

As for the BIOS setting you mentioned, set that to AHCI if you're using an SSD.  You don't need RAID, and definitely do NOT use ATA, because that causes the system to emulate legacy IDE, which will noticeably reduce the performance of an SSD.

So, to recap:

- Set your system's SATA Operation to AHCI in the BIOS.

- Install your SSD internally.  Do not try to install Windows while it's connected through an adapter, because even if Windows Setup allows that (which it might not since Windows does not allow itself to be booted from a USB device, except for Windows To Go installations), if you try to install it to a drive that's connected via USB on a UEFI system and then move it internally later, the UEFI firmware won't have a valid bootloader file path registered in its firmware for the new location of that disk.  UEFI systems boot very differently from Legacy BIOS systems when it comes to internal storage.

- Build regular Windows 10 installation media using the Microsoft Media Creation Tool.  Stop using this Easy2Boot thing.

- Boot that install media in UEFI mode, open Command Prompt by pressing Shift+F10 or Shift+Fn+F10, run diskpart's "clean" command, and then step through Windows Setup normally.

im sorry for the confusion.

goal was clean install to win10(UEFI) after win7(NOT UEFI) was running.

easy2boot is just an external to install win.iso. so lets call it just installing win10 in uefi mode from an USB.

then i changed the SSD with a conventional HDD and it worked like a charm.

the SSD is working but just not performing the conversion from MBR to GPT. is it possible the controller is old or whatever?

Some SSDs become read-only when they’ve reached a wear level that they can’t reliably write data anymore, but if that was the issue I would expect even the clean command wouldn’t work. You can install Windows 10 in UEFI mode from a USB flash drive without any special tools, as I just explained. You admittedly can’t boot directly from the ISO — you have to actually extract the contents onto the drive — but again that’s easy.

The steps I provided above in the recap portion of my post work. If you encounter an error even doing that, then please provide exact details about the error, although in that case I would suspect a hardware problem with your SSD unless the details of the error suggest user error as a possible cause.

i had that programm installed and that was the awkward thing. every green light was on. sandisk's utility. crystal disk info. SSD sentinel.

everything looked fine. it just could not install windows on it again.

but now its working and i thought just maybe someone had an idea.
PS: i always got some kind of message trying to install windows 10 in UEFI mode on a HDD/SSD which was NOT partitioned in GPT.

@littlepyro  your best bet would be seeing if SanDisk themselves provides any sort of management/diagnostic utility for their SSDs.  Otherwise I believe there are some third-party tools that are generic and may be able to display the error if it's an industry standard type of error or that tool has some special support for SanDisk SSDs.  You really don't need to perform a manual GPT conversion before going through Windows Setup, though.  It will always do that automatically if the disk is currently MBR.  The only reason you'd need to do that yourself would be if you were going to set up a custom partition layout upfront and possibly use DISM to apply the Windows image and skip the Windows Setup wizard entirely.  But of course there's no harm performing the GPT conversion, so do whatever makes you feel better.

i succeeded installing in UEFI mode after i received a new SSD (exactly the same sandisk model). after diskpart's clean and convert GPT command was performed in a second i was able to clone my (old) HDD to the new SSD.

so it has to be the SSD, which was faulty. but i am still curious, which error that was.

any idea to find that out?

@littlepyro  not sure what to tell you about that "some kind of message".  When I set up systems manually rather than using a mass deployment imaging solution, I boot into Windows Setup, press Shift+F10 to open Command Prompt, run diskpart's "clean" command, then exit Command Prompt and go through Windows Setup unless I specifically want to use DISM to apply the image because I want to use custom partitioning.  I've never had Windows Setup when booted in UEFI mode throw any sort of message trying to install to a disk that had just been "cleaned" with no further action -- and since I work in IT and do a lot of tech support for friends and family, I've done a lot of Windows clean installs.