I have SuperDrives installed in all four of my Lisa's, and they function exactly the same as 800K drives. The only benefit is that the SuperDrives are newer and a little more reliable mechanically.
Well, you're using them for at most 800K access so you wouldn't see any difference in normal use. (BTW, perhaps you could document how you've wired them to work with the Lisa's controller?)
However, they are capable of reading and writing ~2x the rate of normal drives, which means that the heads are going to be more sensitive. Now this might be done by increasing the data rate of the shift register used, and/or playing with the speed.
In general some later read/write heads also have an erase coil that runs just before the r/w head - this erase head is going to be wider than the r/w head so as to leave some space around to prevent data bleed to other tracks. So when the heads go off center, you wind up with that drive being mostly able to read/write from its own disk and do a marginal jobs of accessing media written to by other drives.
This gap is what we're after:
Assuming a liner track, it might look like this:
trk1 trk2
| gutter| data | gutter | data | gutter |
| gutter| data | gutter | data | gutter |
| gutter| data | gutter | data | gutter |
| gutter| data | gutter | data | gutter |
Now when trying to do data recovery, we don't care about the erase head at all, and the width of the track, we just want to access the data:
Here XX's show mechanical damage to data:
| gutter| data| gutter |
| gutter| data| gutter |
| gutter| daXX| gutter |
| gutter| daXX| gutter |
| gutter| XXXX| gutter |
| gutter| daXX| gutter |
| gutter| daXX| gutter |
In this second case, using a more sensitive head, and purposefully forcing the drive head to go off center, can recover the "da" data bits, but not the XXXX case. This is a moonshot ofc, damage is not always going to be offcenter, and could span multiple tracks depending on the geometry of the damage. But you might be able to get some more data out of it than not.
And yes, I am suggesting that you modify the drive to get at the data. This is called microstepping. This is a step too far perhaps for some as it will render that drive usable only for recovery use. Also, you may need to change this in both directions - once to be slightly to the left, then try to read multiple times and see if you get different data, and then once to the right, again, with repeated attempts to read the damaged areas.
This will give you a whole bunch of data that's going to be different (and fail checksum). So next you'd have the massive task of trying each of those passes to see if any got you further or not.
If you take a look at the opposite problem, on how fully destroy magnetic media, you see that multiple write passes are required with various patterns. This is so you remove any left over signal. see:
https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html In conventional terms, when a one is written to disk the media records a one, and when a zero is written the media records a zero. However the actual effect is closer to obtaining a 0.95 when a zero is overwritten with a one, and a 1.05 when a one is overwritten with a one. Normal disk circuitry is set up so that both these values are read as ones, but using specialised circuitry it is possible to work out what previous "layers" contained. The recovery of at least one or two layers of overwritten data isn't too hard to perform by reading the signal from the analog head electronics with a high-quality digital sampling oscilloscope, downloading the sampled waveform to a PC, and analysing it in software to recover the previously recorded signal. What the software does is generate an "ideal" read signal and subtract it from what was actually read, leaving as the difference the remnant of the previous signal. Since the analog circuitry in a commercial hard drive is nowhere near the quality of the circuitry in the oscilloscope used to sample the signal, the ability exists to recover a lot of extra information which isn't exploited by the hard drive electronics (although with newer channel coding techniques such as PRML (explained further on) which require extensive amounts of signal processing, the use of simple tools such as an oscilloscope to directly recover the data is no longer possible).
Using MFM, we can go even further than this. During normal readback, a conventional head averages the signal over the track, and any remnant magnetization at the track edges simply contributes a small percentage of noise to the total signal. The sampling region is too broad to distinctly detect the remnant magnetization at the track edges, so that the overwritten data which is still present beside the new data cannot be recovered without the use of specialised techniques such as MFM or STM (in fact one of the "official" uses of MFM or STM is to evaluate the effectiveness of disk drive servo-positioning mechanisms) [7]. Most drives are capable of microstepping the heads for internal diagnostic and error recovery purposes (typical error recovery strategies consist of rereading tracks with slightly changed data threshold and window offsets and varying the head positioning by a few percent to either side of the track), but writing to the media while the head is off-track in order to erase the remnant signal carries too much risk of making neighbouring tracks unreadable to be useful (for this reason the microstepping capability is made very difficult to access by external means).
Now the difference between using highly specialized equipment and doing the above is that reading a 0.95th of a "1" signal is that we lose the indication that it was 0.95 and not 1. A fact which might have proven useful when doing data recovery. In other words, we don't have access to the analog levels of the bit strength which might provide a clue.
Imagine the magnetic media gets scraped by something, but not fully all the way down to the plastic part of the cookie. So maybe, think of it as a road with a pothole, but it hasn't gone down all the way to sand. If the road were magnetized, there might still be 25% of the media there, and its orientation is still readable.
But because the circuitry past the head has a discriminator that wants a whole 1 or whole 0, you cannot read this "1" bit and get a zero bit instead. This is the big issue here. But if we had access to analog levels with some degree of sensitivity, the original data might be recovered.
OFC, when the road is scraped all the way down to the sand layer underneath the concrete and asphalt, there's no way to get at the direction of the tiny magnetic fragments as there are none to see.
So this would give you maybe a 10%-20% chance of improvement.
Now if you were to build your own analog floppy controller and make it very sensitive, well, that would increase your chances a lot more. This is very difficult and would need a lot of processing power as well as a lot of code to write, but would allow us to recover a whole lot more. You may also need to do other things like read the data a lot more slowly, etc. and have some way to distinguish between different types of damage.
And ofc, the unmentioned thing here is dirt. Cleaning both the drive heads and the media is useful. However we have to be careful to not dissolve the media or damage it by doing this. I'm not an expert in this area at all, so not going to comment on IPA vs other substances, and leave that to those who have done it before. It's certainly possible to clean the media and then read it and recover it as well as causing permanent long term damage such that it won't be readable again some time in the future. This is going to be the same argument for/against retrobrite on cases, etc.
Final edit: all this said, this is theoretical, I personally don't know if you can modify a 2M Sony SuperDrive so the track is slightly off center, perhaps someone here has done that repair and can help describe it.
