I also have this same Interior Hard Drive Error, only with the MRS-1266. I contacted Zoom to learn that all they could do was either sell a new hard drive to me for $500 (list) or have them install a new HD for $200 + $25/hr. Not being able to afford this i had no choice but to try and fix this myself.
The friendly lady at Zoom did confirm that INT HD Error did, infact, "sound like the hard drive is bad." Well, I took this supposed bad drive out of the Zoom and connected it to my PC as a slave drive. Test after test after defrag after file check showed this drive as being in perfectly fine shape. I even downloaded Zooms Music File Manager from zoom.co.jp and now have all my songs & projects saved to my computer (i have no usb or cd add-ons.)
So if it isn't a faulty Hard Drive, what could it be? Has to be the software, right? Wrong, i downloaded Zooms 1.09 update and changed the files accordingly and still.. INT HDD ERR.. So I'm following a process of elimination now. Now I'm on to the cable's. If you notice, you can unplug the power to the disk drive and the HDD light will stay on but the screen and HDD light doesn't "blink" as it does with the power plugged in. So its not that. If you unplug the data cable to the drive, you get no light at all.
So in conclusion, this "INT HDD Error" isn't from a faulty hard drive, power cable or data cable. I still haven't found the solution.
Please let me know if you have any ideas/suggestions. I'm out of ideas.