Mirror command execution incomplete

[Lutz]

I'm just mirroring nearly 1M backup files (900GB) from old 1TB external HD to new 2TB with Sync/Mirror-command from a folder comparision of the old HD against an empty folder on the new HD.
The estimated duration was about 20 hours due to USB transfer speed.
I've about 8 hours to go and saw 18K files in folders already processed by the sync were not copied.
I copied them manually for now within the existing folder comparision while sync is running and it was ok.

1. question: Why aren't all files copied? Is it recommended to use folder sync instead to get more reliability or have I been hasty because the sequence of the copied files isn't the order as shown in the folder comparision (sorted by name)? I noticed that two different folders are mirrored at the same time (using dual core CPU?) and copying seems to be processed from the last subfolder backwards to the first.

Memory usage is above 1GB (of 8GB physical memory).
FAQ:

Folder Compare

Folder Compare capacity is limited by the number of files being compared. The largest comparison we've successfully completed was 2.5 million files on each side. For larger numbers of files BC3 will run out of memory.

2. question: Is there a limitation of memory usage (4GB) and what memory usage has been at the successful test of 2x2.5M files?

Greetings Lutz

[Aaron]

Hello Lutz,

To clarify, the files had not copied over yet, but the copy was still in process? We can use multiple threads to perform copies (certainly over FTP), and do not necessarily copy in alphabetic order. I would suggest letting the copy finish and then seeing if the compare shows any errors or uncopied files.

Later versions of BC3 are large address aware, so we can use what Windows gives us (32bit Windows gives 2gigs, 32bit with a special boot flag can give 3, and 64bit Windows can give 4gigs). All the testing on our KB article was done on 32bit XP.

[Lutz]

...
To clarify, the files had not copied over yet, but the copy was still in process?

Yes.

We can use multiple threads to perform copies (certainly over FTP), and do not necessarily copy in alphabetic order. I would suggest letting the copy finish and then seeing if the compare shows any errors or uncopied files.
...

Ok Aaron, I've been too hasty. I did some other large copies in the meantime and saw that copying don't follows alphabetical order and all of them finished completely.

Greetings Lutz