Setting it to Storage mode you'll be able to flash the new 34xx logo in preboot mode with updater
Download it here:
Logo splash screen for nuvi 34xx hid 1376
or
34xx_350_logo_hid1376.7z
and flash it
After I have flashed the logo, my device is "stuck" on "Software Loading...." with Garmin logo below...

I'm not confident to turn it off and restart, but it has been like this for the past 20 minutes.

Any ideas?