Disappointing but not at all that unexpected either now on checking further. I've been able to source a nuvi 310 to dump it's region 14. It's just not quite big enough to fit the fw_all.bin from 6xx, short by only 135,168 Bytes (132kB or a lousy 0.12 - 0.13 MB) in the words of Maxwell Smart "missed it by that much". More bad news. Mostly all fw_all.bin files can be compressed but unfortunately 6xx is one of only a couple of Garmin's firmware that can't be compressed successfully. Some more explanatory background info, when you flashed the full fw 3xx to your 610 the region layout was resized, a bit like a re-partitioning a drive, so the space available in region 14 became 10,223,616 bytes (~9.75 MB) whereas the 6xx layout is somewhat larger.

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Now, it's logical to think "Why not just flash the full 6xx firmware with SD bin files and re-size back in the process?". So i should explain a little about why flashing the full 6xx firmware might not work using SD. The Ldr.bin (boot.bin) initiates flashing of the main system software (firmware). You presently have 'foreign' 3xx Boot BLK and X-loader in the 6xx device's regions 5 and 43. They were flashed there via an RGN in preboot using 3xx's boot.bin which is initially stored in region 12, a virtual rather than physical region. We are now intending to overwrite those regions with the correct data at the very same time that they are being used too by Ldr.bin to write new data to region 14 at the same time it's attempting to change the region layout resize back to a 6xx device from the present [foreign] 3xx layout. Can you see the possible complication? It's this: Writing to a region while it's being used can be potentially deadly and give you a paperweight.

Still wish to try? I can now only give you one option: Attempt to write the full 6xx firmware to re-set the region layout sizes and change it's HWID back to 0596 (Fingers crossed it doesn't die on the spot). If you want to precede post again and I'll make another SD kit. It seems you have little to lose though by going ahead.

PS: Your dumped nonvol region 41.bin file will be empty, 0 Bytes. That's why it shows an error for it too in update.log file.