Generation 2 Chevy Volt (2016-2020)
[EDIT/UPDATE: So it looks like the Volt disliked the exFAT filesystem I'd originally formatted the drive with. I have reformatted as NTFS, reloaded 19GB of the original 69GB of music, and it works fine, with the drive showing up on the display as (correctly) 128GB. Initial indexing was < 60...