Any make or model will work but each vendor 500GB drive may vary in size so it may be better to get a disk that is over 500GB to be safe or better yet find the same make and model #.

Thanks Tom. So we could replace this 500GB drive with a 1TB one, let it rebuild, and then replace the others with 1TB drives, following the same method? (I'm working on the basis, if one has packed up, the others are likely to follow suit fairly soon).

Not sure what happens to the free space.  Once all are 1TB It may allow you to expend or create another paration.  

Whether you can do this depends partially on the operating system and partially on how the discs are structured.  With XP and earlier you'll need third-party software to do the job unless you're willing to start from scratch and reload everything.

I thought it maybe useful if I let you know how I got on, just in case anyone needs to replace a failed RAID disk on a Precision system running XP and RAID10.

Firstly as suggested I bought a 1TB disk to replace the failed 500GB one. I also bought three more identical 1TB disks to replace the others, on the basis they've all been running the same time and so are therefore likely to fail about the same time too.

This MB uses the Intel Matrix Storage Manager, so I downloaded the latest version from Intel and installed it.

Simply identify the failed drive by port number/serial number and plug in the new one. When you boot into Windows,  hit 'rebuild' in the Intel software and some few hours later the system reports normal.

To replace the other three disks, simply repeat this step with each one allowing it to rebuild each time.

To use the additional space on the drives, again in the Intel software, 'extend' to make the space usuable and reboot.

Download EASEUS Partition Master (free), select the used partition and right click. Hit Resize and pull drag it to use the unallocated space.

Goes without saying do a data backup before messing with drives, just in case.