Yeah. On my end it just slowed to a completely unusable level. So while it technically didn’t lose data it effectively did since I was looking at weeks to replicate any data off it. I let it run for a week and it was still crawling when I gave up (4TB drive)

Was it a SMR drive, by a chance? You can make those crawl with any filesystem. I even had one that couldn't finish initial Time Machine backup.

I just double checked the drive and it is. I forgot about that aspect of it.

However I have run this same drive out of space using ext4, zfs, and xfs playing with it (it’s a drive I use for torrent etc) and none of them have crapped out like btrfs did on it.