A thumb drive is just another name for usb drive. You need a usb drive to put inside your 360 go to system settings then storage, you will see usb storage device press a to select it. It will ask you to configure your usb drive just read the instructions, if you press configure now it will do it automatically for you. Move your Xbox profile from your xbox hard dive to the usb drive then copy your MLB 2k13 roster save from your xbox hard drive to the usb.
Download the horizon program to your computer and install it, then download the roster from here to your desktop unzip the folder and drag it to the horizon program. Then you open the horizon device explorer click were it says flash drive then click games when you see MLB 2k13 click on it your roster save will be there, drag it to the middle of the horizon program now you will have both rosters open in the middle of the screen. Now on the roster you download from here delete all the zeroes you see.
Then copy and paste the numbers from YOUR roster file (which will have letters and numbers showing up in the profile, console, device and ID numbers - you'll see them clearly - right in the middle of the file). paste them into the roster file that you download from here. CLICK REHASH and RESIGN then click save on the new roster to your usb drive.
Now place the usb back into your 360 load the game and then load the 2015 roster from the usb drive.
Now I hope you understand you need to take YOUR file numbers and put it in the one you download.