If I understand you correctly, you can’t open an original text file on your new Mac, but a copy opens fine. That’s odd. Maybe do this: do a get info on your hard drive( command/Apple key& I key) after you right-click on it. See if it’s set to read and write or just read only. To change that, click on the lock at the bottom of the info window. You will, of course, need your admin/ login password( the one you may have used when starting up your Mac, and you had to type it in to get into your Mac. Sounds silly, I know...but....
Save the changes and close the info window( red x at the top of the window) .. do the same get info stuff for TextEdit, and set that to read& write, etc.... close the window and try opening an original, other than that, maybe zapping the pram/resetting the pmu/smc might help-and there’s disk utility that’s worth a try( you can find it under the utilities folder somewhere on your hard drive)
Good luck
John B