Question, why do you need a floppy disk to boot up your 510? Do you actually mean that you need it to load software or do you really need it to boot up? If you actually must have it to boot up something is wrong.
I have had similar issues to what you have mentioned. 2 of our 5 wire machines have the same control as your 510. The floppies themselves do seem to fail far often. Currently we download programs via RS232 instead of using the floppies for just the reasons you have mentioned. Although RS232 is not as nice as the floppies are (when they work correctly) it is simple and it does work. If you want to go that route I should be able to help you. You don't need any special software and many programming systems come with a DNC application. We use Windows Terminal. If I remember correctly it may have been part of windows 95. Not sure. It is just 1 .exe file with a help file that accompanies it. I keep telling myself I will get something else but I never do. Don't confuse it with Hyper Terminal.
1 drawback. You still need the floppy drive in working order to be able to reload software. You can't do that with RS232. Supposedly you can load the technology files off of the software disks via RS232 although I have not been able to. So we have working floppy drives that we use only to reload software when necessary and .tec files when we need something out of the ordinary.
In theory you can/could buy the same floppy drive from Teac. I found it listed on their website although they tell me it is out of production. Have seen a few used ones on the net but they are always gone when I follow up.
Haven't heard of anyone getting a high density drive to work. I have been told that the issue is not as much with the drive as with the machine software. Supposedly the software does not know that high density drives exist and therefore can't recognize them.
These controls were pre USB so no luck there either. Would be nice though.
I have purchased brand new still wrapped Verbatim DD floppys by the case on E-bay. Just keep your eyes open and be patient. (they still go bad too often though)
Updating the control would be a nightmare. The control not only makes the machine move, it runs the EDM generator, all other machine functions, and provides the interface for us humans to communicate with the machine. This control is not flashy (and yes it graphs programs slowly) but it will do anything the machine tool is capable of doing.
How long have you been running the 510? If you are used to Japanese machines it will seem quirky until you get used to it.
Hope this info helps. If you would like any help with the RS232 just let me know. I am not too crazy about it myself but we use it because it works.