How do I remount a filesystem as read/write?

The correct syntax is:

sudo mount -o remount,rw /partition/identifier /mount/point

Where mount/point is /partition/identifier's corresponding mountpoint, as listed by the following command:

mount -v | grep "^/" | awk '{print "\nPartition identifier: " $1  "\n Mountpoint: "  $3}'

For example, say that the above command gives this:

Partition identifier: /dev/sda1
 Mountpoint: /    

Partition identifier: /dev/sda2
 Mountpoint: /boot

Partition identifier: /dev/sda3
 Mountpoint: /test

The following would be the correct syntax. (We start by unmounting it, if it's already mounted.)

sudo umount /test
sudo umount /dev/sdb3
sudo mount -t hfsplus -o rw,remount -force /dev/sdb3 /media/untitled

for busybox/android users:

Oddly, I needed to add a space (in contrast to normal usage) between 'remount' and 'rw':

mount -o remount, rw /

otherwise it wouldn't work.

UPDATE: it seems that this is almost never the case (see comments). Not sure which busybox-version + android-version I was running. I'll just leave this here in case anyone still runs into it.

Running dmesg | grep hfs showed that the filesystem was unmounted incorrectly, which I was able to repair using

fsck.hfsplus /dev/sdb3/

First, let us fix NTFS problems (if you have an Ubuntu/Windows dual boot setup)

sudo ntfsfix /dev/sda7

Before mounting we need a Directory (folder)

mkdir ~/Desktop/disk

Now mount the partition

sudo mount /dev/sda7 ~Desktop/disk

In this case "sda7" is the partition name. Now you read from and write to the partition.

I have Dragonboard 410c I connecting via adb I wanted to mount the physical sdcard as RW. the following worked for me.

adb root
adb shell
su
mount -o remount,rw /storage/sdcard1   /storage/sdcard1

So I can now access it in rw mode as /storage/sdcard1

nb. /storage/sdcard0 is emulated and is /sdcard