NO! NO! NO!
Let me tell you about my experience. A few years back, I recorded an episode of ER on my DMR-E80H. I missed the start of the show, but I happened to be recording the same channel on my new Zenith XBR413. In order to make a complete episode for archiving, I figured I would copy the start of the show to the hard drive of the DMR-E80H. So, I fired up my PC, imported the video that I recorded on the XBR413, and wrote it back out to DVD-RAM. (You cannot copy from DVD-R to the HDD on the E80). When I tried to copy the video to the E80's hard drive, it didn't work the first time. I should have taken the hint. After a few tries, it finally worked...sort of. Although the title showed up in direct navigator, it could not be played or copied. In fact, I couldn't even delete it!
So now, 3 years later, I still have this corrupted title on my hard drive. It just sits there, taking up space. I suppose one of these days I'll get everything off the hard drive and reformat it.
When you "make a decrypted copy," you're re-encoding the movie on your computer. The DMR-E80H may or may not be able to handle the new bit rate/compression/whatever. If you try to put stuff on the HDD that was not recorded on that recorder (or similar), you could corrupt the hard drive or worse.
BOTTOM LINE: Don't do it! It's not worth the risk. Since you're importing the video into your computer anyway, why not do the editing there?