I learned something while repairing a customer's two TRS 80 Model 4P computers. If you have 128K installed and the system does not boot (gives you CRC error during POST of the OS disk), try removing the back 64K RAM (remember to move the jumper back to 64K mode) before you assume it's the disk drive.
There have been times when I thought the CRC error was always due to disk drive not being able to load the OS, when it was actually the RAM in error. It's a lot easier to find and replace a bad RAM chip, even if you have to do it one at a time, than find a working disk drive. Hopefully this tip with help someone else.