DELL PowerEdge 1850 HDD Upgrade

"for the rebuild work the replacement disc will need to be the same size and speed as discs that are allready part of the RAID array?"

not  true

For Lsilogic , Adaptec and most other raids..... for the rebuild to work, a disk introduced into a raid  must be either the exact same size (that is formatted size),  or larger; speed can be different thought it is not recommended for the replacement disk to be slower. I state formatted capacity as I have run into disks of the exact make/model/capacity  which would not work in a raid due to firmware revisions which had formatted capacities being different by a couple megabytes.

Sounds like a bad replacement disk

Seconding what pcmeiners said ... drives must simply be at least as large as existing drives in the array (even a few MB is enough to cause the controller to mark it as unsuitable).  

What RAID controller are you using?  The onboard RAID controller is a PERC 4e/Si - an LSI controller, not Adaptec - and all certified add-in RAID cards are LSI.  Are you seeing this in CTRL-A or CTRL-M?  How are you "attempt[ing] to add it to the RAID group"?  If an LSI controller, you must "rebuild" a "failed" drive (you can also assign it as a hot-spare).  If this is the case, then I agree ... sounds as though the disk is bad.

BTW ... if you are using a certified RAID controller on this machine (LSI), then you should know that you cannot make your array bigger by simply replacing the disks one at a time with larger disks.  Once completed, your only option will be to create a second array across the disks using the new unused space.  This will show up as a new drive letter in Windows, and as it is physically separated, there will be no way to re-arrange (grow) your existing partitions to use the new space.

You are right - it should rebuild automatically, but there are several things that can cause it not to - in that case, you can start a rebuild manually, in either OpenManage (in the OS) or in CTRL-M.  Just remember, you must Rebuild, NOT force online.

OpenManage is a suite of management utilities for Dell servers.  OpenManage Server Administrator is installed on the OS as an application and interfaces directly with the system hardware, allowing you to monitor and configure hardware logs, BIOS settings, RAID configurations (including individual disks), etc.

The "key" it is speaking of is a security key, presumably to act as the security certificate for running OMSA (it uses an https web server to display the pages).  OMSA does not (cannot) come with a pre-installed security certificate.  In Windows, you must generate a certificate from within OMSA after it is installed (also optional, as you can simply click on the "continue to this website" link in IE) ... it would seem that is not the case on Linux.  In either case, downloading/installing/running/using all OpenManage software is free - consider it included with the server.