Replace the hard drive if you have a spare or replacement available. A spare part number is available on the drive's label. The ML370 G5 is possibly out of standard warranty (3 years) at this point, so you may have to purchase a replacement. The part number on the disk will help there.
You can download and install the HP Management agents for XenServer, which would give you greater insight into hardware health going forward. In the meantime, you could reboot and enter the BIOS raid configuration utility (press
F8 when prompted) to see the Array Controller Configuration. However, depending on the array health, that may not be a good idea.
How did you identify this problem? HP drives will show a solid amber or red light upon failure. A flashing amber/red light indicates a disk pre-failure. What do the LEDs on the disks look like, and how many drives are present in the system? That will help us understand what's going on.