I have added a harddisk to my desktop system, but I am not allowed to write to it as user.

Here is the output of 'id' and 'groups' from userarea and root, respectively:

$ id
uid=1000(myself) gid=1000(myself) groups=1000(myself),6(disk),7(lp),10(wheel),21(locate),81(dbus),82(hal),91(video),92(audio),93(optic al),94(floppy),95(storage),98(power),100(users),102(policykit)

# id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),19(log)

$ groups
disk lp wheel locate dbus hal video audio optical floppy storage power users policykit myself

# groups
root bin daemon sys adm disk wheel log

All my disks have

User: root
Group: disk

I had thought that as long as the user (myself) belonged to the group 'disk', I, as user, would have full/all permissions. And I do, for the disk which was present during my Linux installation (Arch Linux), but not for the newer disk. I have the newer disk commented out in 'fstab', but that shouldn't make a difference? Otherwise all settings look, to me, to be the same for all disks.

What am I missing?

1) With the new disk commented out in fstab, the new disk will not mount on boot (assuming you have partitioned it). If it's not mounted, you can't write to it.

2) You don't give any indication that you have partitioned the new disk, and which filesystem you chose for it.

Points taken.
I can mount and read the files on the disk using e.g. Dolphin, but I don't have write permission as user. From the command line I have full access/permissions as root.

The filesystem is JFS. The disk is 1.5 TB SATA, not partitioned. I also use JFS on my /home and /data partitions, on another (ATA) disk, without problems, and I use JFS on an unpartitioned 1.0 TB USB external disk without permission problems.

The size and the Serial ATA implementation are the only differences I can think of from the disks I am using or have used before...