With the IIfx, those large drives tend to suffer from stiction - when the lubrication on the heads, patters and spindle solidify into a thick gel and causes the controller to burn out. So that drive is going to need to be replaced. And take out its mother board and check the two silver caps on the extreme right side that tend to leak. I'm replacing the ones on my IIfx. I also need new PRAM Batteries and a way to jump-start the IIfx as well as a new hard drive.
As for the color classic, I have seen that certain OSes tend to soft-crash (lose boot information) more than others. If you can get a Norton Emergency Boot floppy disk or a System Tools Disk for that Color Classic, you should be able to run Disk First Aid and 8 times out of 10, fix an otherwise dead drive. Just make sure that the floppy disk's OS is the same or above that of the Color Classic's hard drive OS. For me I have fixed more "Dead Hard Drives" this way than actually taking the machine apart. In a worst case you should be able to format and reinstall the OS on the hard drive.
As for the color classic, I have seen that certain OSes tend to soft-crash (lose boot information) more than others. If you can get a Norton Emergency Boot floppy disk or a System Tools Disk for that Color Classic, you should be able to run Disk First Aid and 8 times out of 10, fix an otherwise dead drive. Just make sure that the floppy disk's OS is the same or above that of the Color Classic's hard drive OS. For me I have fixed more "Dead Hard Drives" this way than actually taking the machine apart. In a worst case you should be able to format and reinstall the OS on the hard drive.