Your comment on USB-SATA cables is interesting. I don't recall whether the serial numbers were identical before I changed one of the discs - they had run happily together for more than a year. My concern is to ensure that it is not the identical serial numbers which are causing my disc to disappear and become unallocated.
Entering your proposed command I get:
-bash: armbianmonitor: command not found
Meanwhile I get this with hdparm:
hdparm -I /dev/sd? | grep 'Serial\ Number'
Serial Number: WD-WXK1A10X4320
Serial Number: WCC2RCP3
So the system sees the real serial numbers whilst the OMV GUI gives different serial numbers, identical for both discs (even when using different USB-SATA cables). That serial number is 116AC2101219.
As I say, my concern is to avoid a repetition of the vanishing disc. If identical GUI serial numbers are a non-issue, I'll try again with moving all of my data on to the new disc.
Incidentally, it shouldn't be a power supply problem as I run everything from a powered hub and have had previously had three cohabiting discs - I only had two discs when the new one became unallocated.