Why Can't I Copy Multiple Files From One Portable Hard Drive to Another on my MacBook?

Usually the issue is one of the three problem-types below

1. External storage device’s permissions not allowing Read and Write sharing

     -> need to reset permissions

          -> run Disk Utility

               -> check First Aid tab

2. File system disparity could lead to two particular problems

     - Cluster disparity between external device and Mac

          -> storage space wasted

     - Files over a certain size can’t be transferred

             e.g. MS-DOS FAT 32 is limited to files  <=4GB

     -> run Disk Utility

          -> check First Aid tab --> check file system

3. Either Mac or external device may be infected with a virus or worm etc.

     -> Run antivirus e.g. Trend Micro, Norton or Kaspersky

However, for problems 1. and 2., often the only way to reset permissions, to allow both Read and Write, is to

     -> run Disk Utility

          -> check Erase tab

               (1) Back up the data to be transferred 

                    (can take time and be frustrating, but must be done)

               (2) Set file system to ExFAT

               (3) Erase data on external drive and reformat

-> should be able to Read and Write

     -> transfer backed-up data to original location