The sim is always installed on the C: (or boot) drive by the MS/Asobo download program (MS Store or Steam).
Did you “Clone” your C: drive?
If not, FS2020 will not know where its programs are.
If your C: drive and Window 10 is okay:
Go to Settings, Apps and under MS Flight Simulator, click “Uninstall”.
Go to MS Store or Steam and install FS2020 again.
if you have the “Installed Packages” on your D: drive, those files will not be downloaded again.
And only the 1.xx GB will download to your C: drive.
If the download pgm can’t find the “Installed Packages”, it will download them again.