I have around 18, yes it works...
Don, still outdoing me. I only have 12--5@.75TB, 4@1TB, 1@1.5TB, 2@2TB. Four are on stands, one or two depending if one is used for computer backup.
This is for 2 722s and a separate 622. You may have all connected through a USB hub or all powered up but only one powered up and connected per DVR. Currently I just pick the color-coded USB and power block to use for each DVR.
Occasionally, just this last week for the first time in a couple of years, you may have to ask for an authorization hit if a known good disk causes the DVR to request a re-format of the EHD. This will change the Menu+6+3+Counters+pagedowns Household Key to change, so have all DVRs on the account ready for the hit. Curiously, 2 have the same number and the other is similar.
-Ken