I have had several Dish DVRs, with internal and external drives. External drive is formatted in proprietary file system (probably some Unix variant), and cannot be accessed on an average PC. And even if you can, the recordings there are encrypted, and accessible only on your DVR.
Once upon a time (when DVRs were first introduced by Dish) some guy has managed to "crack" the recording scheme, and extract videos, bbut this was 10+ years ago.
If you just want to reclaim an external HDD for PC use - clean partition table, create new volume(s), format and use.