Something I've just noticed while putting together a zdb -l dump of the disks through USB vs ESATA - the pool indicates a resilver in progress. Perhaps it might be wise for me to let that finish before I try ESATA again.
In the meantime, ESATA:
http://pastie.org/5081597
The same, but through USB:
http://pastie.org/5081612
The only thing I immediately notice different is the hostname.
ESATA:
hostname: 'Expanse@6BED3659-3A55-5A92-8909-174E2CAF490A'
USB:
hostname: 'ZFS_ON_DARWIN'
I'll let the resilver finish and see if that helps.