suddenly a read only file system

[pittendrigh]

I searched for this topic. Didn't find it.

I have a Mint Linux Cinnamon 18.2 installation now several months old.
Two hard drives. Main is a 3tb drive partitioned so /home mounts on /dev/sda4, so I can change the OS as needed, without erasing data. 6tb /disk2 mounts on /dev/sdb1, to which I back up /dev/sda1 and /var/www/html (my development area on localhost) using rsync.

No disk is filled up. / is 88% full. Perhaps I should have made that partition bigger.

Three or four days ago, in the middle of running rsync my file system became "read only" even though each disk and each partition still had space. I got to single user mode and ran fsck and got it to boot.

This has now happened a second time, also during a large rsync.

sudo smartctl -a /dev/sda1 | less seems to look OK.

Do I have a hard disk or a software problem? How to tell?

I have backups. Perhaps a re-install from bare metal.
Linux dungeon 4.8.0-53-generic #56~16.04.1-Ubuntu SMP Tue May 16 01:18:56 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

This is an aging Dell box with lots of ram.

[WharfRat]

What does df -h show