A zonal isolation device (10; 20; 30; 300; 400; 500; 600) in the form of an active external casing packer is provided which includes a tubular section such as a casing or liner (1; 21; 31; 171; 203) arranged to be run into and secured within a larger diameter generally cylindrical structure such as an open borehole (163; 213). At least one sleeve member (3; 23; 33; 43; 173) is positioned on the exterior of the casing or liner and is sealed thereto and at least two deformable band members (201; 401, 403; 501) are provided around and are bonded to the outer circumference of the sleeve member. The sleeve member and the at least two deformable band members is/are expanded into place against the inner circumference of the open borehole by increasing the pressure within the sleeve member. The method detailed herein also provides zonal isolation by use of a pair of spaced apart sleeve members during a frac operation where frac fluid is supplied to a zone (in between the pair of sleeve members) requiring to be frac'd, where the frac pressure acts not only on the outside of the zonal isolation device but also on the interior of the sleeve member to enhance the seal provided thereby.