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added to section multiple Yubikeys section re: switching between Yubikeys

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section describes the issue and the remedy for GPG stubs only pointing to the Yubikey that was last subject to the keytocard command
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@ -1529,6 +1529,31 @@ $ cp -avi /mnt/encrypted-storage/tmp.XXX $GNUPGHOME
$ cd $GNUPGHOME
```
## Switching between two or more Yubikeys.
When you add a GPG key to a Yubikey using the *keytocard* command, GPG deletes the key form your keyring and adds a *stub* pointing to that exact Yubikey (the stub identifies the GPG KeyID and the Yubikey's serial number).
However, when you do this same operation for a second Yubikey, the stub in your keyring is overwritten by the *keytocard* operation and now the stub points to your second Yubikey. Adding more repeats this overwriting operation.
In other words, the stub will point ONLY to the LAST Yubikey written to.
When using GPG key operations with the GPG key you placed onto the Yubikeys, GPG will request a specific Yubikey asking that you insert a Yubikey with a given serial number (referenced by the stub). GPG will not recognise another Yubikey with a different serial number without manual intervention.
You can force GPG to scan the card and re-create the stubs to point to another Yubikey.
Having created two (or more Yubikeys) with the same GPG key (as described above) where the stubs are pointing to the second Yubikey:
Insert the first Yubikey (which has a different serial numnber) and run the following command:
```console
$ gpg-connect-agent "scd serialno" "learn --force" /bye
```
GPG will then scan your first Yubikey for GPG keys and recreate the stubs to point to the GPG keyID and Yubikey Serial number of this first Yubikey.
To return to using the second Yubikey just repeat (insert other Yubikey and re-run command).
Obviously this command is not easy to remember so it is recommended to either create a script or a shell alias to make this more user friendly.
# Cleanup
Ensure you have: