Error -36 when copying files

I am unable to copy video files from one external HDD to another. The Folder containing he files starts copying, then stops and gives the message: "The Finder can't complete the operation because some data in "AVCHD" can't be read or written.

No information is copied over (zero KB in the copied Folder).

Permissions seem to be okay, and I'm able to copy the files from the video camera's SDHC card to the desktop without issues (and import them for editing, etc).

I've never had this problem in 1.5 years with this laptop / camera / external HDD combination, nor have I updated any software recently.

I always try to have a backup of my backup (so two external HDDs with identical files / folders).

Thanks!