Mounted partition is suddenly read only

[JD]

I have a 1.5TB drive partitioned into 5 partitions.
partition 3(ext3) has in it the dir home.

/home is a symlink to /sdb3/home

The rest of the mounted partitions are
/sdb1, /sdb4 /sdb5.  /sdb2 is a swap partition.

I had left the system idle for many hours.
When I returned to it,I found my home dir
was no longer write-able.
So, I tried:

# mount -o remount,rw /sdb3
and got

mount: cannot remount block device /dev/sdb3 read-write, is write-protected

None of the other sdb partitions are affected in this way.
To wit:

# for i in 1 3 4 5; do
 > [ ! -d /sdb$i/tmp ] && mkdir /sdb$i/tmp
 > touch /sdb$i/tmp/junk$i
 > done
touch: cannot touch `/sdb3/tmp/junk3': Read-only file system

The other partitions are not affected.

I googled this and tried several of the solutions (which do not require 
reboot).
None of them worked.

Rebooting into rescue mode, running fsck and then going multiuser
works, but only for a while (i.e. until I leave the system idle for a 
while),
and the issue repeats itself.