Typical behaviour of failing mechanical hard drives.I went to access a big file in one of my folders. It wouldn't open. Tried copying it to another part of the drive. Couldn't. Tried copying it to an external drive, couldn't. Tried a file next to it. Same thing. Tried files elsewhere on the drive, these were OK. Downloaded and ran DriveDX. This told me that the drive was failing due to read failures (bad blocks, etc). Tried to do a repair in Disk Utility but couldn't. As my machine earns me my living I cannot have work files being saved to the hard drive without my being able to trust that I can open them later. So machine had to be opened up and fixed, pronto. Used my 2015 Macbook Pro while I waited.