I tried several USB cables on various Pi's and a VM virtualbox (Debian) on Windows 10.

[Superlub]

I tried several USB cables on various Pi's and a VM virtualbox (Debian) on Windows 10.

All ending with the usb_bulk_send() error -7.

Maybe this is relevant:

https://github.com/linux-sunxi/sunxi-tools/blob/master/fel_lib.c
{
/*
* With no progress notifications, we'll use the maximum chunk size.
* Otherwise, it's useful to lower the size (have more chunks) to get
* more frequent status updates. 128 KiB per request seem suitable.
* (Worst case of "slow" transfers -> one update every two seconds.)
*/
size_t max_chunk = progress ? 128 * 1024 : AW_USB_MAX_BULK_SEND;

size_t chunk;
int rc, sent;
while (length > 0) {
	chunk = length < max_chunk ? length : max_chunk;
	rc = libusb_bulk_transfer(usb, ep, (void *)data, chunk,
				  &sent, USB_TIMEOUT);
	if (rc != 0)
		usb_error(rc, "usb_bulk_send()", 2);
	length -= sent;
	data += sent;

	if (progress)
		progress_update(sent); /* notification after each chunk */
}

}

Originally posted by @Superlub in #15 (comment)

[AtomSub]

Did you follow these steps?
Remove the C.H.I.P from its case (in case you have a Pocket C.H.I.P).
Connect the FEL and a GROUND pin of the C.H.I.P (for example, with a paperclip).