yes you can fit a 3.5" drive in a amiga 1200
as i have done it you were once able to get
a kit from eyetech you need 2.5" 40 pin Amiga
motherboard to the 3.5" 44 pin i think i used ide fix
but you still need the cable powercomputing
do a nice little bufferd interface which fits
neatly into the a1200 iam not shure but i think
you will not need the cable part of the kit was
bit of plastic for glueing to the bottem of the hard
drive to stop it shorting on the amiga metal
casing and power supply adapter
GOOD LUCK :-D :-D :-D