Time Machine decided to misbehave. I’m still running OS X 10.7.5.
mount_hfs & syslogd were consuming close to 100% CPU time (on this machine with 2 cores), so it was clearly grinding the system to a halt. I couldn’t kill the mount_hfs process (kill -9), but I could kill syslogd though it would repeatedly restart.
Problem was only solved via power cycling. There was advice from Macworld titled: One fix for a runaway syslogd process, but nothing from that was useful.
Root cause seems to be another external disk going wonky. Yes, I’ve seen 2 disks die being Time Machine backups in a span of what, 3.5 years? Worrying trend.
root 30064 88.0 0.0 2434804 776 ?? R 10:40am 2:45.62 /sbin/mount_hfs -u 99 -g 99 -m 755 -o nodev -o noowners -o nosuid -o owners /dev/disk1s2 /Volumes/Expansion Drive