Now that I had play the external storage for awhile, here are few things that I have found:
I did a transfer with 80 recorded programs (each with 175MB), no problem from 622 to USB (WD MyBook 500GB). Watched from USB, no problem either.
Then, I made a restore from USB to 622, it stopped at third program ans indicated there is an error on USB. I skip this program, and re-selected the rest of programs, transfer completed without problem. The one didn't transferred, still play back without any problem from USB.
Few things that could improved: a) USB should have the same sorting function as DVR; b) the program list does not show the date/time of the recorfing, if you have many same named program, you have to select the program to find when was recorded, as on DVR, when you scroll through the programs, it will show you the date/time.
Other than those little things, so far so good.