Find the tixati app data folder where it stores it's config.  This is usually c:\Users\YourUserName\AppData\Roaming\tixati
From one of the forum threads. If you had a portable Tixati, it's in the same folder as the .exe file.
Backup these files and then experiment with the new Tixati install. I think you should just copy over all .dat files. If your external HDD is on D: again then it should find everything automatically. If not, I suggest you change the drive letter.