Assuming you have a JVC MiniDV Camcorder, the E04 error message means that the rotation of the video head has stalled or is too fast.
When the little motor that pulls the videotape through the camera detects that the tape is not moving at the right speed, you get the SAFEGUARD MODE error. The principle here is that the camcorder detects a problem and shuts down in order to avoid doing additional damage to the unit. This is a legitimate way of protecting itself.
The problem is that there are many reasons why the unit might not be able to move the tape, and obviously the question is: Why it is happening to my unit in particular? If your unit is still under warranty, then by all means run to JVC and have it diagnosed and repaired for free as soon as possible. Once your warranty runs out, if you start getting these errors again, it can cost over $150 just to have JVC look at it. Then possibly several hundred more to repair the unit.
A few Quick Fixes to try:
In most cases you should clean your camcorder's heads by running a cleaning cassette through.
A little trick some have tried to eliminate this error message is to keep the cassette door open and then pull out the battery. Doesn't work? keep trying this routine. Some people actually report getting the error message to clear.
Eject the cassette and just when it is about to eject, push it back in... and the error is gone.
In Depth Solutions:
Solution #1: Cold Reset
The reason it tells you to REMOVE AND REATTACH BATTERY is not that the battery is bad, but that removing all sources of energy (battery and power cord) causes a COLD RESET. When you use the unit's on/off switch, parts of the camera are still receiving power and so you do not get the benefits of a COLD RESET. Just like your car; turning the ignition off will still power the clock and radio. You have to completely remove all power. If you are lucky, the camera will reset itself and start working again.
Solution #2: Awareness of Humidity
It seems older units would issue the same SAFEGUARD MODE error message for more than one problem. For example, older units do not tell you that they are shutting down for an hour because they detected high condensation, or moisture. So you play with the camera, remove the battery, reattach it, remove the tape, reinsert it, stand on your head, pray to your god, go to the bathroom, and voila! The camera is working again. It was nothing you did, just that the humidity went down enough for the unit to start working again. So consider high humidity as a possible source of the problem. If it is a rainy day, or you are near the sea, you might never get it to work that day in that location.
Solution #3: Head Cleaning
If you have never used a head-cleaning tape on your camcorder, you might give that a try. Mine has come back to life a few times by doing just that. Just don't do it too many times in a row because you can permanently damage the head. Read the instructions.
So we have covered a few reasons why these errors come and go as if by magic. They are intermittent and recurring. The camera will appear to work at times, and the next minute it shuts down.
Solution #4 : Fake Solutions
Another reason why the problems come and go is that, as several technicians have reported, the problem can be a loose connector inside. That is why some people report that tapping the camera gently or rocking it side to side has brought it back to life. Some have gone as far as putting cardboard between the tape door and the tape mechanism in order to exert pressure and keep the camera working. I can only warn you that these kinds of "repairs" are temporary and likely to damage the unit further.
Solution #5: Compressed Air
On some models, there is no shield covering the many delicate electronic and moving parts in the tape mechanism when you open it to replace the tape. A piece of dirt, grain of sand, or other debris can get caught in the tiny gears and stop movement or throw them off alignment. You can explore this possibility by getting a can of compressed air at any computer shop and blowing it into the inside of the camera. Follow the instructions, keeping the air nozzle however many inches away from the target as they say. Practice on some other object first, because if you tilt the can or hold it too close to the target you can freeze the target and do more damage than good. This applies to fingers too!
Solution #6: Tape Age, Brand, and Type
Have you tried using a brand new tape? Or only your really old ones? Try a new tape.
Now for the controversial issue of what brand of tape to use. Controversial because JVC's own Stacy Sample, Service Coordinator for JVC in Houston, TX says that "TDK and Maxell tapes tend to be two brands that are very problematic in our digital line wherein Panasonic, JVC and Fuji seem fine. We just find that TDK has the most problems, Maxell is a close second and Sony is 50% of the time.
A different point of view I ran into is. Yes, there is an issue, but it has nothing to do with the brand, but with the tape type. There are MP and ME tapes. These two types use different particles, and different lubrication. The two lubricating materials form some sort of goo when mixed together, if you use both types of tape in the same unit. So as long as you don't mix tapes of different types, they will work just fine in your camcorder. Some refer to these types as "dry" and "wet", but I don't know which is which. In any case, it is not known (to me) how accurate this point of view of not mixing tape types is.
Solution #7: Do not store tape in unit
Because of the delicate nature of the miniDV mechanism, do not store the unit with a tape inside. The theory is that a loaded tape will put pressure on parts of the mechanism and increase the chance that parts will stick together, making the tape harder to be pulled by the motor, causing the SAFEGUARD error.