In show etherchannel summary, the EtherChannel shows as "SD" (down). How do you troubleshoot and fix this?

Check the output for flags, compare the channel-group mode on both switches, and fix any mode incompatibility.

Troubleshooting workflow:

Step 1 — Check summary:

S1# show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator
Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports