Thanks, yes, a power cycle can help in this case.
Generally the Disk Surface test window shows disk drives / volumes exported to the operating system, as only those drives/volumes could be tested by the surface test. So (for example) if you'd just connect a drive in the enclosure and/or you'd just connect a drive to a RAID controller, then it is possible that the drive can be recognised (its health/status displayed) but the enclosure/controller may not yet make it available for the OS. Then the surface test window does not show the corresponding drive.
Similarly, if you'd have an unconfigured / hot spare drive on a RAID controller, then Hard Disk Sentinel will likely detects its health, temperature but the surface test window will not show it (however, Disk menu -> Short self test / Extended self test can test it, depending on the controller). But this is an other question