I had exactly the same problem today, with my sg-3100.
Luckly, I also had a backup config and a 2.4.4 p2 firmware in a pendrive.
Installed, upgraded, restored, download blacklist from squid/pfblocker, restored my keytab file to squid folder, and everything is working perfectly again.
I will perform the reboots from now on with the F option, to force fsck.