rsync --verbose --archive --dry-run /oldisk/a/ /newdisk/a/
The --dry-run (or -n) will do a dry run, showing you what it would do without actually doing anything.
If it looks ok, run the rsync without the -n option.
This will be a copy, not a move, which isn't quite what you're doing, but is safer. The --archive (or -a) ensures all the ownership and timestamps metadata is preserved (which a regular copy would not).
