I have to say that due to our 'efforts' Garmin has started to protect their devices from flashing patched firmwares. In particular, GCDs for nuvi 24xx, 34xx, 30, 40, 50 are digitally signed. There is a 256-byte signature appended at the end of every GCD. The signature is an encrypted SHA1 hash of GCD contents. I can claim that SHA1 is used, because I've seen it in disassembly. Also I'm quite sure the encryption is asymmetric because long-integer functions are used. So it looks like we can't fake the signature. Now imagine that Garmin digitally signs also RGNs and update.txt&ldr.bin on some new device. The era of easily hackable devices would end.
That's why you'd better think twice before sharing things like UnitID copying.