Yes, I bricked one 34xx while flashing it with 37xx firmware. But that time I used 37xx boot.bin and also I messed up something with HWIDs. Maybe this was the reason.
As long as 34xx boot.bin is used and NV and the internal flash drive are backed up, it should be possible to restore nuvi 34xx to its original state.
Also 34xx bootloader should work with 37xx firmware, as it's prepared to launch a larger 34xx firmware.
But other problems could appear. For example, it seems that 37xx firmware supports less GPS chips than 34xx firmware.
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