USB-C external HDDs not recognised by Mac via Apple Digital AV Multiport Adapter

When I plug USB-C external HDDs into either iMacPro or Mac Book Pro, using a USB-C to USB-C cable via Apple USB Type-C Digital AV Multiport Adapter (i.e. USB-C to USB-C/HDMI/USB-A), they are not recognised by the Mac. Have others come up against this?

When I plug the same external HDDs using a USB-C to USB-C cable into the Mac directly, they work fine, but since Apple supplies only 4 USB-C ports and no other types on a MacBook Pro (and one of those is gobbled up by power) I need to use adaptors to connect non-USB-C peripherals; external monitor, RAID, backup disks, USB Hub (card reader, Wacom, colorimeter, printer, scanner). So, even if my peripherals did use USB-C, the meagre 3 available ports are insufficient anyway.

However, if using USB-C to USB 2 type A, or USB-3 Micro-B to USB-A, each plugged into same adaptor, the HDDs are recognised fine - except of course for losing the speed advantage with an expensive USB-C HDD.

I do not believe it's caused by any of the following:

1) Faulty Apple Multiport Adapter adaptor. I have two, same issue with both. I initially thought (incorrectly) that the problem might be due to one being faulty, so it was replaced. So, that's same issue with three adaptors.

2) The Mac. Same problem on 2 x MBPs and 1 x iMacPro.

3) The O/S. Currently using Mojave 10.14.5, but I've had this issue ever since the arrival of USB-C (so through various O/S). Have not yet upgraded due to lack of support for 32 bit software on latest O/S - and because Apple unable to tell me this is a known fault resolved by upgrade. (Does anyone know?)

4) Anti-virus software (suggested by Apple, but I think we were clutching at straws here... I really doubt both MalwareBytes and BitDefender would differentiate between cable connections and take a specific dislike to USB-C (...would they?). I've used both at different times.

5) The external drives. It affects 3 different drives; 2 x 2019 La Cie external USB-C drives and 1 x G-Drive SSD; problem is the same on all 3. ( I have not therefore tried contacting the individual hardware suppliers, as also suggested by Apple, but again clutching at straws IMHO).

6) Cables (yep, tried changing them).

Does anyone have a solution to this please?