4TB HGST Ultrastar 7K4000 shows as 2 TB with Dell PERC 6/i RAID controller

I went through much pain with this, and the bottom line is that although the controller supports array expansion, Dell have disabled this in their Array Manager and they will not support it. If you replace the Dell driver with the LSI one (I think the controller is an LSI, though I wouldn't swear that it isn't an Adaptec) and use the LSI management software then you might be able to do it. However I would find this a scary business!

My solution was simply to create a new logical disk in the free space. This is completely safe and needs no reboots, though obviously it's clunkier having separate disks. If you really, really want a single partition then you'll have to backup then delete and recreate the array. Note that if your server has the Dell utility partition on it you need to be careful with partition imaging tools as you need to recreate the Dell utility partition or your server won't boot. I speak from (bitter) experience.

JR

PERC 6i is LSI gear, so in general anything that goes with the MegaRAID SAS controllers, will work on the Dell PERC.

We can add a disk to the array one at a time, and the controller will expand the array

You will notice this not to be the case. Just adding the drives will make them available for the use in a new container, but not automatically grow anything. You should be able to use the Dell OpenManage tools to add the disk to the array as a replacement for a defective or offlined disk - the array will regenerate by writing redundant data to that disk.

What we do not know is what happens when ALL the drives have been replaced, and are now 450Gb drives, does the RAID controller automatically grow the array to use all 450Gb on each of the disks or will we end up with 50Gb wasted space on each drive?

At first, it is the latter - your array will be the same size with unused space on each of its disks. But you might try to expand your virtual disk using OpenManage Storage Manager. Please note that the expansion of the virtual disk will not expand your partitions and filesystems contained there. You will need additional software to do that - a GParted live CD should cope with most resizing needs and will support your controller out-of-the-box.

will the controller allow us to then create a new RAID 5 array using these 6 x 50Gb spaces

Yes, but you should only do this if the disk expansion is infeasible for some reason.

With all that said, it is a complex operation which might fail and take all data with it at any stage, so having a backup which can be restored swiftly (e.g. a full disk image) is strongly advisable.