512 GB card arrived: check
file system set up: check
music collection synced to it: check
Now, what am I missing? 
In case it comes up, I was reminded once again of the annoyances / delights (you choose) of ext4lazyinit.
This will particularly be a problem with people initialising larger cards. (Cards will arrive with an exfat file system, which almost certainly is not what anyone would want. So setting up the file system is the first thing that you will have to do.)
The symptoms of ext4lazyinit (which is the default) are that the file system creation will complete almost immediately … and then a system process called ext4lazyinit will grind away for eternity actually initializing the inode table, the light on the card reader (if you have a light) will flash away interminably, and you have no progress indicator, no way of knowing how long until it finishes, … and these cards are not fast.