Subject: Re: Very OT: Help with Mac-Win problem?
... I know that a lot of the active posters are computer geeks ...

There are quite a few more over at the Mechanical Investing board. As with here, folks are generally helpful if a post is marked OT.

Jim


A couple of random thoughts, cribbed from one of those evil AIs, and re-interpreted by me and therefore probably even less reliable.

Allegedly, MacOS and Windows use different default allocation bock sizes on large drives (Mac smaller).
It could make a difference which OS was used to format the partition for that, or other reasons. Rumour has it that formatting under Mac is more compatible
Allegedly, there are two types of allocation tables, GUID/GPT and MBR. The latter is sometimes more common on external media, and sometimes more universally compatible. Check which was used.
So maybe format it again using the macOS Disk Utility, ensuring you use the MBR partition scheme?