Maybe it is due to the fact (https://www.smartmontools.org/wiki
/Supported_USB-Devices) that USB connected disks usually need a '-d'
option to enable S.M.A.R.T. support.
In fact, for the same disk, trying first:
$ sudo smartctl -d scsi -H -i -A -l error -l selftest '/dev/sdb'
gives "SMART supp
I confirm this bug on Linux Mint 18.1 with gnome-disk-utility 3.18.3.1
[UDisks 2.1.7 (built against 2.1.6)]
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https://bugs.launchpad.net/bugs/1540110
Title:
gnome-disk-utility (shown as