I have always transcribed disk when it pertains to the back and disc when it pertains to the eye.
Another medical transcriptionist told me today that she always had to use disc in her reports because her facility followed the Terminologia Anatomica reference.
I was curious so I looked it up in The Book of Style for Medical Transcription, third edition, and here is how I understand it:
Most medical resources use disk other than for ophthalmologic use. An exception would be Terminologia Anatomica uses disc referencing to spinal terminology (more on that is on page 393 of Book of Style).
I will continue to use disk for back reference and disc for eye reference unless of course the facility I am transcribing for tells me otherwise because they have the last word. Hopefully, that makes sense (if not send me a message).