So EHD’s from VIP DVRs (612/722/922/etc) can be used on Hoppers because the file structure is the same, and the only data saved on the EHD is recorded events.
211/Wally’s on the other hand use the EHD for more things, like the 9 day extended guide (2 day on internal memory) and the live TV buffer (the ability to pause/play/rewind up to an hour). This makes it so that Hopper to Wally/211 are not interoperable, and for some reason the 211 and Wally file structure is different.
I don’t have a Wally available to me right now, but something I would try if you really want to transfer the recordings is what was mentioned above, after connecting a new EHD to the Wally and letting it format and create its specific file structure, connect both EHDs to a linux computer (the EHDs are formatted in ext3 iirc) and see if you can transfer the recordings over. You would just have to be careful to not transfer something that is not a recording.