66i not talking to pc

This may be due to your choice of interface mode. Main Menu > Setup > System > Interface. I know that Garmin Serial enters mass storage mode when plugged in. Garmin Spanner prompts (on the device) to ask if you want to enter mass storage mode. You need to tell it to do so (sorry, I don't remember the wording of the prompt, but it is clear when you see it.)

To eliminate any ambiguity here, you can select MTP. This uses media transfer protocol, not the "removable USB drive" protocol. The effect is the same in Windows Explorer - you see the device as a drive. The difference is that you don't need to worry about dismounting the drive before unplugging the device. In fact, you will not see the connected USB device icon in the system tray.

Ditto this problem: I had to use the 'special' usb cable that came with the item.

There is nothing magic about the cable that came with the device - other than that it is guaranteed to be a data transfer cable.

USB cables come in two flavors - data transfer and charge only. Typically, the cables that come with devices which CAN connect to a computer are data transfer cables. Cables which come with power banks, wall warts and similar devices sometimes are charge only. Because the manufacturer can save a few 10ths of a cent per cable by using only 2 wires instead of the 4 needed for data transfer.

To successfully connect to a PC, you MUST have a data transfer cable.

Personally, I discard/recycle any charge-only cable I happen to acquire. Avoids confusion and frustration later.

You just solved my problem

Thank you for this information, you helped to resolve this problem that I've been stuck with for many months. I went into the Garmin 66i GPS menu, selected "Setup" / "System" / "Interface" ... just as you noted, and I observed that it was set to "Serial". I changed that to "MTP" and that solved the problem. Now the Garmin Express software on Windows 10 is able to recognize the GPS via the USB serial cable. Thank you again!

Add me to the list of grateful recipients of your instruction. Thanx so much! ... solved my problem as well. 

Solution is to check the cable - charge only cables are common and worthless. Avoid MTP system setting as offered below, it is very slow compared to garmin serial, assuming you are connected with the right cable! Maps update would have taken 16 hours instead of 40 min if I kept MTP in use.