I started to have this problem a week or 2 ago even on start up / open of a new document or small document, or the tiniest edit of an existing (small) document.
As one user suggested, I checked my available HD space and I have more than 15%, although closer to 23%. I am currently deleting some unused apps and my trash folder. However, that doesn't seem to be the problem as I haven't installed anything recently to cause the sudden change.
But I followed your suggestion and here is what I see for my HD.
Sharing and Permissions:
"You can only read."
??? ***? When / how did that change by itself? and how do I change that?
Below that (but greyed out probably because of the lock on settings - which again - should not have allowed any unauthorized, without requiring a password of changed settings unles I was either hacked or duped into giving my password for what I thought was some other setting / need) is:
system: read and write
wheel: read only
everyone: read only.
No clue what "wheel" is unless for some reason it is my husband's profile as he is a car guy. But even that doesn't sound like him. Should be under his first and last name as is my profile.
I should have the same settings for my profile as system admin. Looks like it is "reading" me (logged in under my name as it is shown in the upper right corner of my MacBook Pro correctly) with either "wheel" or "everyone" permissions.
To test it, I unlocked the padlock to change them & changed wheel to read and write & it did not change "my" settings as listed above. I set it back to "read only" and changed "everyone" to read and write, and only then did it say "You have custom access." ?? So it is reading me, logged in under my one & only (& original profile with original admin read and write access") as "everyone." ??? How do I fix this now and prevent it from occurring? Obviously I hesitate to make it "everyone" as read and write because I don't know what that will do. (Does that mean everyone who logs into my actual computer or everyone who has access to our shared password protected wifi (in our home)?
Was I hacked?
We have a password protected wifi here and as most of my computer files are either genealogy files or photos, security is really only a huge concern as far as things like accounts and passwords when I pay a bill and such, maybe our saved tax filing files. But I also don't want someone accidentally changing or deleting a file either such as from the wifi access or if a file is somehow inadvertently shared or something.
Even though I've had the mac for years, I am still ignorant how to do certain utility type scans / protection kind of things like one would do on a PC and how to find certain things that wouldn't be in the main settings or searchable by file name or in the applications. (in other words, no idea how to find hidden files or logs).
This is my sole laptop, though a few months ago, I did allow my husband to create a profile (where he logs in under his name & pw) so he can use it to pay bills or when I am otherwise not using it. He very rarely does so & most often just uses mine.
Thanks