I have created an account just for this issue. I looks like the issue is still there on the latest iPad OS (15.2).
I have faced similar issue this week, during our ski trip in Austria. I have a GoPro 10 and I brought only 2 128GB SD cards with me, each day we shot 20GB-100GB of videos. So I also brought a Samsung T5 SSD for backing up videos. The T5 SSD is formatted to exFat because I want it to be usable on both my macs, iPad, and Windows PC.
I have a 2021 iPad Pro with me, so I brought it to use it as a simple computer during our trip. I used an Anker usb c hub for backup. So my I connected the usb hub to my iPad, and inserted SD card to hub then plugged in my t5 to hub as well. Sometimes I need to try a few times to let the iPad to recognise the disk. Then I used Files app to copy all the videos directly to T5 SSD.
During the first few days it was okay, all files are copied properly and we could view the videos from T5 on the iPad. On the third or fourth day of our trip, after copying around 80GB of video, I realised that most of the videos of the day before was corrupted. Some of them are showing zero bytes, and the others are not playable anymore. More than 50% of the video are corrupted. See the screenshot when I later checked on a computer.
View attachment 1944731
Only the videos with proper thumbnails are playable, those with VLC icons or zero bytes size are not playable at all. So as you can see most of the files are broken.
I still don't understand what was the cause of this issue, to me it looks like the new copy only affects files from the day before. Older files was not corrupted. And only copying large amount of file (> 50GB) will affect older files.
During the trip I thought my T5 was broken, but actually it works fine. I am wondering if there are working solutions for that.
I saw the recommendation of FileBrowser, but it's not free I think, not even sure it will solve my problem. Formatting my T5 to APFS/HFS+ will make it not usable on my Windows PC, I prefer not to do it.