ローカルの容量を食うのは死活問題なので、ディスク未接続時にスナップショットをローカルに作る機能を停止した。
sudo tmutil disablelocal
man pageに記載あり
TMUTIL(8) BSD System Manager's Manual TMUTIL(8) NAME tmutil -- Time Machine utility SYNOPSIS tmutil verb [options] DESCRIPTION tmutil provides methods of controlling and interacting with Time Machine, as well as examining and manipulating Time Machine backups. Common abilities include restoring data from backups, editing exclusions, and comparing backups. Several, but not all, verbs require root privileges. ...中略... VERBS Each verb is listed with its description and individual arguments. ...中略... disablelocal Turn off local Time Machine snapshots and trigger automatic cleanup of accumulated local snapshot data. Requires root privileges.