Volume suddenly read-only

I, too have a volume that went read-only.  A reboot clears it till a write operation occurs, then it goes read-only again.  Interrestingly, the BTRFS readonly flag is set to false.

I happen to know what caused it on mine.  The volume is in an EDA500 equivalent eSATA chassis and the cable came loose (it's the older shallow kind, not the newer, deeper one).  Though the connection is OK now, the damage has apparently been done.  Since this volume is fully backed up and consists of archival data rarely accessed, I decided to try a few things.  But it sounds like you may not have tiime to wait for any answers I find.

I thought a scrub should fix the issue, so I started one.  It self-aborted after 4 minutes.  But initiating a scrub from the GUI also initiates a re-sync, and I have a few hours left for that to complete.  Note that the GUI still says it's scrubbing, but SSH tells a different story.  That the re-sync process seems to be doing fine tells me that the eSATA connection is now OK.  But re-sync on an eSATA volume is even slower than a normal one, and so I wait.

Once the re-sync completes, I'm going to try a scrub from SSH so it won't also do another re-sync.  If it then works, maybe the re-sync really was needed.  If it still doesn't work, then I wasted a lot of time when I could have been destroying the volume, re-creating it, and restoring data from backup.  While StephenB  suggests a fctopry default, that's not the appropriate action for an external volume, but may be the only total solution for a single chassis system to insure the OS and swap partitions also don't have issues.  But after all this, I'll know, and I can share my experience.