That is a... pretty damning thing to have in your user manual. I've owned many cameras over the years, and I don't think I've ever seen a manufacturer hedge basic functionality like this.
> Data transfer is not guaranteed in the following environments, even if your computer is equipped with a USB port.
>> Computers with a USB port added by means of an extension card, etc., computers without a factory-installed OS, or home-built computers
Manual for version 1.7 page 286
https://download.omsystem.com/pages/inst/om1/manual_om1_v1.7...
Never use the camera over USB, the experience is terrible on everything from Canon to Sony to Panasonic to Fuji.
Don't fight it, just buy that $30 USB hub and get on with your day.
My older camera only had a usb 2 connection over a special connector, so I used to pop out the card. But the OM1 can also charge the battery via the usb-c connection, so it's an all-round great experience: I don't have to fiddle with random parts, not forget to put them back in, etc.
Never had a single image corrupted among dozens of thousands of them. Funnily enough, my older camera used to complain about the SD card every other time after plugging it into my mac. It would shut up after a format. That card is still going strong.
I transfer the files with lightroom (but don't remove them from the card) on a windows pc.
I'm no longer even a semi-pro photog, but these days I even just plug in a USB-C SD reader into my iPhone and upload them that way, it's quite nice really. No corruption yet for me, but I only shot a thousand photos on my trip to Italy, and few other than that these days!
I’m an old Unix guy. Files are files. Card to computer. Process later.
A better way to have further narrowed down the problem down to Actually iPhoto would've been to do the same tests with a USB-C card reader plugged directly into the Mac, which would've eliminated cables, hubs, and camera hardware/software/firmware as possibilities.
It's worth noting that searches show that OM-1 USB support is imperfect, the camera manual addresses that "USB transfers aren’t guaranteed in some setups", and user consensus seems to be to use a card reader for reliable file transfers.