The default installation path for Windows NT is drive C:.
Unattended setup does NOT have any special functionality to change this because the Windows NT setup program, WINNT.EXE, already supports the /T switch.
The /T switch specifies the target location for the temporary files and operating system files.
Assuming that you map the the /I386 folder to drive X:, the following scommand line will install Windows NT to drive D:
x:\winnt /b /t:d: /s:x:\ /u:x:\unattend.txt