Did it cause a problem?
Were you able to see the data in the partition?
If I read you correctly, you have a (USB, firewire?) drive which appears as---eg /dev/sda. You then mounted one of the partitions to a directory:, eg:
mount /dev/sda1 /xxx/xxx/directory
This is not going to cause a problem unless there was data in /xxx/xxx/directory. In that event, the mounting would obscure the data (until you UNmount) the external partition.