Data Control Policy - "Please copy the file using Windows Explorer"

Hi All,

I've got a Data Control Policy with several "Allow file transfer" entries to Removable Storage, or Optical Drive or Floppy Drive.

I have one "Block Transfer" entry..

For any file
where the file contains:
10 or more matches of Credit or debit card numbers with qualifying terms [Global],
and where the destination is
Removable Storage
or Optical Drive
or Floppy Drive,
Block transfer.

Whenever this is enabled, I'm unable to copy anything from the network to USB storage regardless of the content (i.e a Powerpoint Presentation with a single unrelated picture inserted will be blocked).

I get an "Attempt to write "*file name*" was blocked. Please copy the file using Windows Explorer." We are copying the file using Windows Explorer (keyboard shortcuts and right-click Copy, right-click Paste cause the same issue). The same occurs if I copy to the desktop first and then to the USB storage. If I un-tick that one rule, I can copy files without issue.

Is this the expected behavior? I've read various posts I've found in my searches but nothing seems to explain this particularly well. 

Sophos Enterprise Consol 5.2.1 R2 / Sophos 10.6.3 VE3.64.3 Client

If anyone could point me in the right direction it'd be greatly appreciated.

Paul K