the volume has the wrong case sensitivity for a backup?

What are you trying to copy? A whole set of backups? A single backup? A single folder? A file? Are you copying via drag & drop, or trying to restore via Time Machine?

Presumably, you're trying to copy something from your Time Machine backups on your Time Capsule. If so, that's case-sensitive (Time Machine does that automatically); most likely, your external HD is case-ignorant. If you're not sure what that means, see #E7 in Time Machine - Troubleshooting.

Please clarify just what you're trying to do, and we may be able to help.

selinameng wrote:

I try to drag and drop the imac backup in Time capsule to my portable hard drive.

Are you dragging the entire set of backups? The sparsebundle, or the Backups.backupdb folder inside it?

tried again, it's stops again and with msg of "you portable hard drive media is in use and can't be powered off" I have no clue what this means. Do you know why?

That means something else is trying to use the drive. Quit as many apps as you can, eject and disconnect the drive, wait a moment, then reconnect and try again immediately.