Will this compromise the DVD I was writing? The actual application showed a drop in write speed, then it returned to normal.

The actual lock-up only lasted about 5 seconds. I've got Windows to thank for that, does it every time it powers the HDD back on.

DVD-RW: Pioneer 111D.

You're probably OK but there's no real advantage in powering off an HDD. A drive will tend to last longer if it continually spins , it's the restarts that are more likely to kill a drive.

I only keep mp3s on this drive, so it doesn't get used more than 4hrs or so a day, can't see the point in having it on the other 20hrs.

The disc wrote ok and seems to play ok, I assume the buffer took care of it as it was only going at 2.4x speed?

I understand your reasoning. The ideal in this situation is an external drive which is what I do.

Why the 2.4x burn speed? Was that a dual layer disc?

Yeah dual layer. Presumably if the buffer had run out on it or the burn process had been interrupted in any way it would have failed.

Software used was CloneCD.

No it wouldn't; all DVD burners have mandatory buffer under-run protection, so you can have any number of interruptions to the data flow without the burn failing.

If those interruptions happen constantly however, the burn quality may be slightly worse depending on the drive and firmware.

There is no (consumer) DVD burner with a buffer that can last 5 seconds at any burn speed above 1x. The normal buffer size is 2 MB which will last about half a second at 2.4x. The largest buffer in any consumer drive is 8 MB.