I was able to solve this according to alternative 2 in my original post - use lvm to grow the root file system with a new disk. Here is a brief write-up of how I did this. It might be useful to someone in a similar situation - or anyone wishing to add a disk to an already running SME Server.
The scenario:
You want to install SME Server as a virtual machine. The host has hardware raid, so you don't want to use the (otherwise excellent) built-in software raid in SME server (it would only waste storage area and possibly also read/write performance). You want the SME Server to have more than 2 TB available storage (in my case 4 TB).
The problem:
Current virtualisation platforms (VMware vSphere, Microsoft Hyper-V, Citrix Xen) cannot provide virtual hard disks larger than 2 TB. I think this is due to some limitation in SCSI (can someone confirm this?). The SME installer cannot utilise multiple hard disk, except for redundancy (RAID 1, 5 & 6). Otherwise a software JBOD or even a RAID 0 could have been an alternative.
The solution:
Install SME server on one disk. Later add another disk and use lvm and resize2fs to grow the root file system to utilise both disks as a single device.
The details:
1. Create a virtual machine with one 2 TB virtual hard disk. Since this virtual disk resides on a hardware raid array, it is already redundant (unless the array is a RAID 0).
2. Install SME Server with the command "sme raid=none". IMPORTANT! If you don't specify "raid=none" the SME installer will create a RAID 1 device with only one disk, and the whole procedure will be somewhat more complicated. You will then have another abstraction layer in addition to the ones assumed here (virtual hard disk, partition, physical volume, logical volume, file system).
3. When the installation is complete and your new SME server is up and running, power down the SME Server and add a second 2 TB disk to the virtual machine.
4. Boot the SME Server and login as root.
5. Create a partition on the new disk (/dev/sdb) of type "8e" (lvm) using fdisk.
6. Add the new partition as a physical volume using the command "pvcreate /dev/sdb1".
7. Add the new physical volume to the volume group "main" using "vgextend main /dev/sdb1".
8. Extend the logical volume "root" using the command "lvextend -L3999G /dev/main/root". The number "3999" should be adjusted to reflect the combined size of the /dev/sda2 and /dev/sdb1 partitions in your system. If you give a number that is too large you will simply get an error message and can retry until you specify a size that fits on your partitions.
9. Power down the SME Server and reboot using the installation CD to the rescue system by typing "sme rescue" at the boot prompt.
10. Choose "skip" when offered to mount the SME system.
11. Activate the logical volume using the command "lvm vgchange -a y main"
12. Extend the file system using the command "resize2fs /dev/main/root". You may have to run "e2fsck -f /dev/main/root" first. These commands take some time to complete, especially if your disks are large and/or slow.
13. Type "exit" to reboot and start SME Server normally.
14. Check the size of the root file system by typing "df -h" to verify that you have been successful.
The reason why steps 9 - 11 and 13 are necessary is that the version of resize2fs that comes with SME Server does not support online resizing of ext3 file systems. Supposedly resize2fs version 1.39 can do this. SME Server 7.51 comes with version 1.35. SME Server 8.0 beta5 comes with 1.39, but I have not had the time to test if online resizing works there yet.
Any feedback to this write-up is welcome!
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