I have a HP ProCurve switch with 24 ports.
If I set up port 24 to mirror port 1 (which goes to my router), can I use port 24 for normal LAN access as well as to run something like ntop or bandwidthd?
Or does mirroring mean that the port can only be used to read the mirrored packets? Meaning a 2nd NIC is required for LAN access?
(source: spiceworks.com)
Your suspicions are correct in my experience. Every mirrored port I come across is a read-only copy of another port; the mirror port cannot also be used to participate in the network, and you'll need a second NIC to do so.