i would try to bifurcate the debugging process.
I) Does the drive read?
a) I haven't had a plus in a long time but it only had a single sided drive, right? So are you using a 400k disk (I'm not sure if it'll read an 800k disk, I'd assume it will - but not a 2HD disk).
b ) Are you sure there's valid system software on the disk?
II) If the answer is I) No, a) Yes, b ) Yes, then I'd default on Uniserver or someone more knowledgeable with the hardware.
Do you have another Mac you could test the drive on (one that has I guess a 400k controller; I don't know if the 400 and 800k controllers are compatible); or do you have another 400k drive to try with this machine?
I) Does the drive read?
a) I haven't had a plus in a long time but it only had a single sided drive, right? So are you using a 400k disk (I'm not sure if it'll read an 800k disk, I'd assume it will - but not a 2HD disk).
b ) Are you sure there's valid system software on the disk?
II) If the answer is I) No, a) Yes, b ) Yes, then I'd default on Uniserver or someone more knowledgeable with the hardware.
Do you have another Mac you could test the drive on (one that has I guess a 400k controller; I don't know if the 400 and 800k controllers are compatible); or do you have another 400k drive to try with this machine?