Some of the drives at a place have the DSD drive in them but they're getting 26, 18 and 14 errors in flight at times. They may run for a couple of days then go out on retries. Any ideas why?

Look at the fault description, if you have an android phone there's a miprom 21 app that is helpful. I would start with 26 brake contact. Brake could be dragging, not picking properly.
You should get brake adjustment info. If you adjust the wrong things your brake might not hold.

A 26 error is the car was running and the brake either dropped or the brake switch opened. I would check them both carefully. If the brake lining is dragging even a little bit it will cause the drum to heat up and expand causing the error. Also, this error can occur if the safety string opens in flight, sometimes the first thing the CPU sees is the brake switch opening

All good advice above. If all that appears good, I’d also check the mechanical lift of the brake itself re: plunger movement. Fine brass dust will accumulate inside the sleeve and it may develop a dust berm at the center core, preventing full pick, or the brass sleeve is just worn out. Could be they’re just gummies up a bit from past lubrication and don’t lift quick enough before drive enables. Either way, inspect the sleeve and plungers and clean them as well. Measure the distance from the lock nut to the brake body, or count the threads exposed prior to removal. 1/8” pick travel of each plunger should be enough for both shoes to run clear and open/close the micro. Micro switches also can loosen up or fail as well, and wires crimped at the brake relay terminals will actually break in some instances.
You’ll find it.