Hard Disk Monitoring

I have 2 USB HDDs in an external enclosure that are reported by HDS fine when connected to a USB 2.0 port. When moving them to a USB 3.0 port HDS locks up (stopped responding). If I power off the enclosure (or disconnect it from 3.0 port) HDS wakes up and works fine.
Any ideas anyone?

Excuse me for the troubles, but I can confirm that no, HDS does NOT "lock up".
Hard Disk Sentinel is enumerating the devices, waiting for response from the drives - and the enclosure in this case. This is usually quick, but in some rare situations, some external enclosures may require (really) lots of time to respond, sometimes even minutes. During that time, yes, it is completely normal and expected that the software seems not responding (while waiting for the device).
If you disconnect/power off the enclosure - then yes, things work again (which confirms the above) so the issue is related to the enclosure - or its changed connection (USB 3.0 - if the same device works on USB 2.0)

If I understand correctly, when the SAME enclosure connected to USB 2.0, things work perfectly?
If so, please use Report menu -> Send test report to developer option, then I can check the situation, verify the chipset of the USB enclosure (when working) and also I may see the USB 3.0 controller, as it is possible that its driver is related and may need an update.

Also I'd recommend to try the following:
1) completely close Hard Disk Sentinel by File -> Exit
2) connect the external enclosure on USB 3.0 connection (when things "do not" work)
3) launch Hard Disk Sentinel and try to wait some more time. It may take even 4-5 minutes in worst case and if the main window appears, please use Report menu -> Send test report to developer option (so then I can see the time reqired for the initial detection on USB 3.0 and may see errors, timeouts or so - which gives details about what happens).

May I ask the manufacturer/model of the external enclosure you use? As then I can try to check it too, but would be nice to see the developer report(s) on the actual configuration - as then it is possible to check and verify the possibilities about how to improve.

I updated the firmware on the external enclosure (Mediasonic ProRaid HUR3-SU3S3 2 Bay 3.5' SATA Hard Drive Enclosure) and HS is running perfectly now.
Very quick reply from Janos at HS company!!!