I have to conclude that if you like your data, you can’t trust it to HFS+ and the Finder.
Using 10.5.1., I decided to try to consolidate some backups by moving files from one external drive to a second external drive. The operation would ultimately require movement of about 250GB of data.
I thought copying using Finder was safe. I was wrong.
Put simply, some of the files have other ownership, so when I dragged them to the new disk, I am warned that I might have to enter my administrator’s password. Fair enough. But I was never prompted. The operation simply failed.
None of the files were copied to the new disk. But they were DELETED from the old disk. Remember, this is a copying operation between two devices, not a move.
They are just gone. Not so much as a see-ya, later dude, or tootle-pip.
This is data that the US taxpayer has funded collecting via federal research grants. Some of it may not be replaceable.
I came across reports of this bug, but it was claimed to have been fixed in 10.5.1. It obviously was not.
I’ve since discovered that this bug is biting a lot of people: