Neither IDEfix nor Powerflyer destroy the other one's data. If you have the split option enabled with the Powerflyer, IDEfix only sees the first part of the drive, the other parts are left alone. Also a wrong MaxTransfer value does not cause read errors, it does cause corrupted files though. What do you mean by the "ID" value ?
IMHO there is something different wrong with your system. Probably related to HDD cables or power supply or heat.
To help you recovering your data, one has to know the status of the system. "Does not boot" is too weak, what does that mean ? Does it show the floppy disk animation or does it give an error requester like "no dos disk in dh0" or something like that ? Does the HDD spin up and initialise itself ? Are the partitions shown in the early-startup menu ? If you boot from floppy disk, does it show the partitions on the Workbench desktop ? Does it validate ? Is the HDD recognised by HDToolbox ? Does HDToolbox tell you the drive is not initialised ?
Bye,
Thomas