Thank you. That is what I did first, but I just now tried again in case of a mistake. The same thing happened: the program opens, but there are no items, customers, or past invoices. Then I went back into the original hard drive (via File Explorer and the hard drive adapter) and launched it from there, with the same result. Program opens, no data. Searching the problem before I asked, I'd seen an answer indicating a hidden user file where the data would be, but the answer and path were for XP or thereabouts.
My only thought is that in the original hard drive the program can't see the data either, indicating the data is lost or the program doesn't know where to look, because the drive, as a external hook up, is F/ instead of C/ or some similar confusion. That is a wild guess, unsupported by any real knowledge or understanding.