You can use a Coldcard either air-gapped with a MicroSD card or connected to a computer via USB cable.
Hardware wallets are designed to protect your private key from exposure to the internet. Plugging a hardware wallet directly into your computer is an intended method of operation, and safe to do.
However, some users feel uneasy when their hardware wallet is directly connected to a computer because it's unclear precisely what information is being communicated through the cable. To address this, some hardware wallets offer an air-gapped method of transferring information, which can provide some additional psychological comfort.
For an Unchained account, there are three common instances where you will want to transfer information between your Coldcard and computer:
- When you first upload the public key to your Unchained account
- After you build a vault, to register your Coldcard
- To sign withdrawal transactions with your Coldcard
How to use a Coldcard air-gapped with a MicroSD card:
- Plug your Coldcard into a power source that is not your computer, such as a wall outlet or battery pack.
- Use a MicroSD card to transfer information between your Coldcard and your computer. The MicroSD card should have a memory of 32 GB or less. Using MicroSD cards with large memory may not work correctly.
Note: Other forms of air-gapping include transferring information by NFC or QR code. Unchained does not currently support these methods.
How to use a Coldcard with a USB cord:
If you want to use a Coldcard Mk4 or Coldcard Q with a direct connection to your computer, that's fine. You may not have a MicroSD card available, or find that this method is easier for you.
Note: Older models like the Coldcard Mk3 require the MicroSD method.
- Connect your Coldcard directly to a USB port on your computer and unlock it with your PIN.
- Ensure that your Coldcard has the correct settings enabled. Go to Settings > Hardware On/Off.
- From this menu, select USB Port. Ensure that this feature is On.
- From the same menu, select Virtual Disk. Ensure that this feature is set to Enable. Avoid the option that says "& Auto."
- With these settings, if your Coldcard is connected directly to your computer with a functional cable, your computer should detect a new location in the file explorer called COLDCARD. This location can be treated like a flash drive or MicroSD. Both your Coldcard and your computer can put files into this location to be found by each other.