As long as you are not trying to go between a Wally or 211 to the other DVRs or reverse you are fine.I didn't think you could use the same external hard drive on all three receivers. I know if you try to use an external hard drive from say the Hopper on the Wally it will reformat the drive, before you can use it.
And then to prove my statement I went into my EHD and found recordings from early 2011, long before the Hopper existed.As long as you are not trying to go between a Wally or 211 to the other DVRs or reverse you are fine.