It worked. The lesson is, when adding archival data, do not merely use a separate index, but ALSO configure it similarly to the settings for the "main" index found in defaults/indexes.conf -- one hot buckets and a longer than 90 day-range worth of data arriving all at once is a recipe for disaster.
Also, when your archival data is very old, adjust quarantinePastSecs to avoid your oldest data being quarantined.
http://answers.splunk.com/questions/712/put-data-in-separate-index-based-on-timestamp
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