The box appears on power up … can’t read drive. Had that many times before and it never has been a big issue. Click on Disk Utility, click on the drive and then Repair and a minute later I would be up and running. Well Apple decided that for their protection that caused problems so in El Capitan you get the utility you see above, one that doesn’t work. No more repair, First Aid doesn’t function. You might say get DiskWarrior, no joy there either. So at this point, all my images from the last week are “locked” up in a drive I can’t access. The most common solution, return to Yosemite and abandoning El Cap. Oh, the joys of the computer at times.