Thanks for your message and the information.
Sounds very interesting. Of course if there is no RAID solution, the drives should be never "linked" this way
(and even in RAID configuration, the status of one drive should never affect the other).
Not sure what happened there, but would be nice to investigate.
If possible, please use Report menu -> Send test report to developer option in latest Hard Disk Sentinel 5.01.13 when both disk drives connected.
This version can be downloaded directly from: https://www.hdsentinel.com/beta5/hdsentinel_pro_setup_50113-trdds.zip
This way it is possible to check the actual situation, examine the drives, their reported status and check what can be wrong. Maybe related to the disk controller or its driver (which can cause headache but can be fixed by a driver update from the Support -> Driver Zone section).
From the developer report, hopefully I'll see things better and this may give thoughts about what happens.