Q: why is my fan running non stop at a high rate

Run an Apple Hardware Test or Diagnostics, whichever is applicable:

https://support.apple.com/en-us/HT201257

https://support.apple.com/en-us/HT202731

It would be helpful if you indicated the EXACT model/year MBP that you have.

Ciao.

It is showing a Hard Drive or SATA cable error.

You can Try resetting the  SMC http://support.apple.com/kb/ht3964

You can do a simple test by booting off a external bootclone to see if the problem is gone.

Are there other gross symptoms besides the fans on highspeed?

It still might be helpful if you indicated the EXACT model/year of your MBP.

Ciao.