|If it's one of the silver-looking bags, it has a metallic inner layer coated with a plastic layer on each side. This is so the bag can't conduct electricity. However, if the outer layer gets cracked, punctured or otherwise damaged things can come in contact with the metallic inner layer and short out.|
If it's one of the pink ones, it's semi-conductive inside and out and is also not a good idea to use.
When I did my HD in the WP, I cut a sheet of heavy 'report cover' plastic to the outline of the drive on the top and bottom and mounted the plastic using double backed tape. No problems after over a year of use, including traveling and other rough handling....