Hello David, the spanning-tree function is likely trashing out the SA500. Assuming the spanning-tree is operating on the SG 300 switch, one of the ports should get shut down as soon as it detects the redundant link on the same vlan.
If the SA500 is locking up or "wigging out" it sounds like the STP is disabled on the Sg 300 and it is broadcast looping, likely consuming all the SA500 memory and making it crash.
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