Hi @tcbetka
Thanks again for your suggestions.
I would have to put the SSD back into the case and wait for a crash to be able to inspect the kernel logs. I might just try that if things get desperate.
The ASMedia Technology ASM1051E SATA 6Gb/s bridge that is detected using lsusb is with the SSD connected externally with the StarTech SATA to USB3 adapter. It ought to be easy for me to do a shutdown, disconnect the external SSD, reboot from a microSD card and run lsusb again to see the identity of the internal chipset. Perhaps that internal chipset is what is being identified as:
Bus 003 Device 003: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter
Just for the sake of completeness, the SSD is a Kingston A400 120GB (SA400S37/120G) which is one that I have seen recommended in this rather useful guide:
I have a busy work day tomorrow and will report back when I can find the time.