question mark after installing new SSD flash

The Transcend SSD when new doesn’t have the OS installed so you do need to install it. The OS you had on your old drive also has a bearing here as the systems firmware is now updated when you install the OS. This can alter how your system will respond when trying to do an internet recovery which I think is what you were aiming for (CMD R). If you were thinking you can access the recovery partition it’s not there yet ;-}

I’ve found it easier and faster just to have an USB thumb drive OS installer drive configured before hand. Then booting up under it run Disk Utility and the OS installer.

You imply it was working before so the question is what happened?

It sounds like you do have a bootable external drive working here what does Disk Utility show you with the internal Transcend SSD drive when you boot up the system with the external drive? Post a screenshot if you can iFixitでの質問に画像を追加する方法

OK, I think I see the problem. The drives firmware needs to be checked and it may need an upgrade. OWC had a similar issue were the SSD did not show up as an internal drive (note the drives Icon is external) So getting it to boot up correctly is an issue. Here's a reference on the OWC issue: A Note on High Sierra Compatibility with Third Party SSD's

Go to this Transcend web site: https://us.transcend-info.com/support/se... or the Transcend European site.

Let us know how it goes!