Thank you for this tip:
"External Drives are under the DVR item on the main FIOS Menu. DVR|Configure + Upgrade|Enable More Storage. I found that the powered-on drive wasn't properly detected even when I rebooted the DVR after it was plugged in. That is, I unplugged the eSATA cable to the powered-on drive and re-inserted it with both the drive and the FIOS DVR turned on. This caused a pop-up telling me to go to the appropriate menu and reboot the DVR again. This time it was recognized"
I just succesfully added a build-your-own external drive. Initially the drive would not recognize, even after numerous reboot/ troubleshoot attempts, etc. Hot plugging the drive as you suggested did the trick. I've only done one brief test recording, but it seems to be working.
WDBGKN0020HN 2TB WD Purple Surveillance drive
Vantec NextStar CX NST-300SU-BK eSATA/USB2 external case
I used the supplied eSATA cable
Only one DVR here also. Motorola QIP7216
Hope this helps someone else.