Hard Disk Monitoring

I have five (5) Mediasonic ProRaid HUR3-SU3S3 external 2-bay drive enclosures connected via eSATA in 2-volume RAID1 arrays to a Windows 10 PC that I use as a fileserver. It works great and I can see each of the two drives reported on each device until I connect a fifth one. I call them units N thru R. Units N,O,P & Q all report their contents except for unit R which simply displays 'greyed-out as "H/W RAID1". If I turn off any other of the four (N,O,P or Q) and then reboot the PC unit (R) will display both drives in it's array. There is no problem when four or less arrays are connected. Invariably, when five devices are connected one will fail to be read. Is this a software limitation or a hardware limitation? Can you please shed some light on this?

Yes, I completely understand the situation.
Generally, there is no limit in the amount of hard disk drives to be diagnosed / checked by Hard Disk Sentinel but yes, some factors may reduce the amount of drives, there may be some limitations based on the actual configuration, devices used.

These external H/W RAID enclosures are built around JMicron chips. These JMicron RAID chipsets used in these external enclosures has a theoretical limitation: it limits how many such devices / chips can be used at any time in any system.
This is what you experienced: only the first 4 RAID arrays (and the hard disks behind these arrays) can be detected at any given time.

Contacted JMicron and offered help to extend and improve the detection in all possible ways as I suspect it may be related (and hopefully corrected) by the firmware of the chips and/or by the programming reference used to detect disk status information - but received no answer since that.
If possible, you may contact JMicron directly about the issue as hopefully things can be improved. Or you may ask Mediasonic and they can contact JMicron about possible updates.

Personally I do my best to improve in all possible ways - but this requires cooperation of manufacturers too...

Excellent response and much appreciated. I shall inquire with Mediasonic for a remedy. Their Jmicron based units are otherwise excellent and I shall also reach out to Jmicron. As you have stated, a firmware update seems to be the most plausible remedy. Hard Disk Sentinel is an outstanding application. I only wish I had made use of it years ago. It saves countless diagnostic headaches.

Do you know which Jmicron chip is used in the Mediasonic ProRaid HUR3-SU3S3?

If I'm correct, JMicron 562 chip used in this enclosure (Super Speed USB3.0 & eSATA GEN III to dual SATA GEN III ports with RAID bridge controller)

Generally yes, there is a limitation of these 56x series JMicron RAID chipsets: only up to 4 can be detected completely.

If you may find older enclosures / RAID boxes with older JMicron 39x chipset, they may work too: as they are independent from these models and work with different kind of detections, these may be working too. With SATA/eSATA 300 MBytes/sec only and may not able to manage very high capacity hard disks, but can be still interesting.

For example, Welland Tera series are such models, offering USB 2.0 and eSATA connection and used these older Jmicron chipsets.
For example http://www.ebay.com/itm/Dual-Bay-2-5-SA ... SwMTZWRUQr
is a such, just for 2x2.5" hard disks, but there is similar model for 3.5" hard disks too.