There should be messages in the router's syslog showing you why the USB drive is R/O.
Yes, in fact, last night my syslog server started to be flooded with it because of the read-only thing:
Server Name: rt-ac88u.homegroup
IP Address: 192.168.1.1
Date/Time: Mar 5 00:23:07
EXT2-fs (sda): error: remounting filesystem read-only
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Server Name: rt-ac88u.homegroup
IP Address: 192.168.1.1
Date/Time: Mar 5 00:23:07
EXT2-fs (sda): error: ext2_free_blocks: Freeing blocks not in datazone - block = 2154679948, count = 1
--
Server Name: 192.168.1.1
IP Address: 192.168.1.1
Date/Time: Mar 5 00:47:11
FAILED to start up
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Server Name: 192.168.1.1
IP Address: 192.168.1.1
Date/Time: Mar 5 00:46:11
cannot open log /tmp/mnt/JFFS/adblocking/logs/
dnsmasq.log: Read-only file system
Could be usb stick failure. There is a pre-mount file checker available to check that drive at boot.
Yes, in fact I ran it twice, once as soon as the issue appeared, and gave me an error message, I then took it to my PC, did a full sector scan and it was ok based on it...
I'm now using another usb drive to see what happens.