Interesting. I'll look into that in more detail this evening.
The Adata must have developed a fault. I put it into the PC, tried to delete the partitions, it hang and timed out - it was fine when it was new. The replacement Transcend works fine in the same enclosure.
The Argon 40 case is quite convenient, but keeps everything at around 40 - 42 degrees idle. I think next time I get hardware for this it will be something better designed for cooling and running 24/7.