Regarding the second issue, it sounds like you may have successfully created the symlink and pointed it to /mnt/usbdrive/Shared (the ln command) either before or after succesfully moving files. That would give the error that those are the same file.
Was the thumb drive previously formatted to fat32 with no other partitions before inserting it into the raspberry pi? This could also lead to the "device not being found"
The 64gb drive should work.
Was the thumb drive previously formatted to fat32 with no other partitions before inserting it into the raspberry pi? This could also lead to the "device not being found"
The 64gb drive should work.