I don't understand exactly your status. Your TB is on old drive connected to separate PC? The profile will be located in the appdata/roaming/thunderbird/profiles folder. Not sure if that's what you're asking.

As the first step, set Windows to show hidden files and folders:

https://support.microsoft.com/en-us/windows/show-hidden-files-0320fe58-0117-fd59-6851-9b7f9840fdb2

Then explore up this path to find your old data:

D:\Users\<user name>\AppData\Roaming\Thunderbird

I'm not sure of the best way to merge that with your current (empty) setup. You could try a wholesale substitution:

(1) With Thunderbird closed, rename the live Thunderbird folder from

C:\Users\<user name>\AppData\Roaming\Thunderbird

to

C:\Users\<user name>\AppData\Roaming\Thunderbird_New

(2) Copy/paste the old Thunderbird folder from

D:\Users\<user name>\AppData\Roaming\

into

C:\Users\<user name>\AppData\Roaming\

alongside Thunderbird_new and see whether Thunderbird accepts the substitution.