HstEx® has the ability of directly accessing at the sector level any physical or logical devices attached to the host system. This allows the user to employ hardware / software write blockers and to recover data directly from a disk or external media.
Do NOT mount images supported image types and then process as if they were physical or logical devices. This makes no sense whatsoever and will result in really slow processing. If the image types is supported, then using HstEx® to natively process the image is by far the fastest and most effective method. If you have an image format that is not supported, please contact support.
A write blocker should be used when accessing physical or logical devices directly. If the hard disk is failing, please use a data recovery imager to capture the data from the device first and run HstEx® against the resulting image.