How do I put a zip in the "internal memory" through fastboot?

If memory serves me, you can't adb push into the root directory. You have to push into a subdirectory.

brkshr said:

Am I supposed to be in recovery still while doing this? It gives me a 'device not found' error

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Run "adb shell". If this doesn't work, you almost definitely have driver issues.

brkshr said:

Same error... It was just working an hour ago to fastboot everything

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fastboot and adb use different drivers, I believe. I usually use Samsung drivers for adb but the drivers that come with the tether app (can't think of the name offhand) for fastboot. Install them in that order and you're good. (see Birdman's stickied guide to rooting to see what I'm talking about)