Run the test. See what errors it shows, if any.
External USB hard disk drives (HDD) are available from a wide variety of sources.
One mail-order vendor I’ve had dealings with (solely as a customer) is OWC, and they deal heavily with Mac.
OWC (also) offers internal storage and internal SSD storage for Mac, if this should come to a replacement.
SSD will be a substantial performance upgrade over a hard disk drive. But more expensive. And again, if.
Amazon and other vendors also offer USB hard disk storage, as do various storefronts.
HDDs with capacity of 4 to 6 TB tend to be the typical capacity range with the best available pricing.
(But with COVID-19, who knows what the prices and availability will be doing.)