[Ryan Mcdermott]

This method worked for me, after figuring out a bunch of details. Hopefully this helps:
1. I read that a headphone cord with volume control helps with sound levels. You don’t need to adjust the volume control, it just helps supposedly.
2. Connect the cord to the line-out on the device with your problem SD card.
3. Connect the other end of the cord to the line-in on another Zoom device with a correctly formated SD card (and enough space to handle whatever you are going to record onto it.
4. On the target device (which for me was a Zoom H1), make sure the recording format is set to the same format as the file you are playing on the source device. (For me it was WAV44.1/16.)
5. On the target device, turn on Auto Level (a simple button on the H1). Also turn on Limiter. Apparently in transfer the cord can heighten the levels from the source file, so this prevents that.
6. On the source device (a Zoom H6 for me), make sure the main recording channel is on (for an H6 that means the 1 and 2 channels). If this is not on, no sound will be coming out of the line-out, even when you press Play.
7. It is very helpful to have a pair of headphones. First plug them into headphone jack on source device to make sure it is playing. Then later when you are recording, plug them into the target device to make sure the sound it’s receiving is what you want.
8. Press play on the source device.
9. Press record on the target device.
10. Use headphones to check that target device is receiving correctly.
11. Record just 20 sec or so and then stop and play the file back to make sure you have what you want. (I learned this the hard way. I had everything set the way I wanted it on the source device, then recorded 90 min. to target device, only to find out that the USB power I was using on the target device caused major background interference. If I had just used headphones to monitor the target device, I also could have avoided this.)
12. As far as I can tell, the target device will just keep recording even after the file on the source device is finished. So if you don’t want a full card of silence, be around to stop the recording once the original file is finished playing.

I hope this helps some others in this dire situation! The final results seem to my untutored ear and eye to be lossless, so this is definitely a good solution.