I haven’t seen fastboot used with a TX1, but do be aware that if you adjust simple environment variables in U-Boot that you can eliminate a lot of probing. There is probing for USB devices, probing for network boot devices, probing for SD card devices, so on…you could just have it directly assume that the boot partition you are interested in is to be executed without probing. Do you have a serial console?
If so, interrupt boot before it gets to selecting the kernel. Type “help” to see a list of commands, or “boot” to continue booting. Pay particular attention to “printenv”, you will see several variables which “boot” in their names. Running “boot” essentially does a macro expansion of some of those…log the U-Boot command from issuing the “boot” command up until the kernel loads, you should see some useful information. Notice that you can edit environment variables and test them as a single time use without this saving into the next boot; but you can also save the variable edits if you like this and it will persist in all future boots.