Sensei supports external S.M.A.R.T reporting, but macOS does not support this out of the box.
You can enable support by installing the following driver:
Note that this driver is not developed by us, it is an open source project from https://github.com/kasbert/OS-X-SAT-SMART-Driver.
(This is the same driver used by other app products for Mac that offer S.M.A.R.T reporting)
By installing it and rebooting, Sensei will load Health report for your external drive.
(Make sure to check in System Settings -> Security for any requests to Approve the Kernel Extension after installing, and rebooting. It may or may not appear).
This driver will stop working in the next version of macOS releasing later this year, which is why we keep this workaround unofficial while we develop our own, future-proof external S.M.A.R.T driver, to be released later this summer.