Well, this is strange... it works flawlessly on my nuvi 1490.
When flashing 097201000999.rgn, try not disconnecting USB cable at all.
It even works when I insert an SD card with the following contents: (ldr.bin is the boot.bin from 097201000999.rgn)
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After booting almost completely the device reboots and waits for a few tens of seconds for USB to be connected.
If I connect USB it enters the flash drive mode, otherwise it boots normally again.