As a (very belated) update to this question: sadly, the powered hub didn't solve the problem. I'm aware there are [post|325124|limitations to the maximum size of replacement HDD which can be installed into the A1409|new_window=true], and I wonder if those same limitations apply to the maximum size of HDD which the device can recognise when plugged into the USB port.
If anyone has the skills, time and inclination to somehow hack and update the firmware to support larger HDDs, I'd happily reap the rewards of your efforts! Otherwise, looks like an 18TB HDD is more than the old girl can handle.
As a (very belated) update to this question: sadly, the powered hub didn't solve the problem. I'm aware there are [post|325124|limitations to the maximum size of replacement HDD which can be installed into the A1409|new_window=true], and I wonder if those same limitations apply to the maximum size of HDD which the device can recognise when plugged into the USB port.
If anyone has the skills, time and inclination to somehow hack and update the firmware to support larger HDDs, I'd happily reap the rewards of your efforts! Otherwise, looks like an 18TB HDD is more than the old girl can handle.
Thanks again to @danj for their response :)