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Yeah, all the Garmins I've owned have worked fine with any external pack I've used. The 1000 was the only tricky one coz you needed an L-shaped USB to clear the bar it was mounted to.
What ancient Garmins are you talking about? You can probably avoid this with a USB data blocker - I've got a couple of these myself. Useful for using public USB charging when you don't want you phone contracting any malware from the "please hack my phone" charging spots.
Nope. I already have external battery packs that cost far less than Garmin's £100 external.