Reusing a drive from an array

You don't need to wipe the drive to do what you're describing. You'll find that you won't even have a convenient way to erase a single disk.

I'd urge you not to break the RAID 1 array as a form of backup, though. What happens if the upgrade "goes poorly"?

This practice shouldn't be promoted because it carries some risk, and doesn't scale to other RAID levels or larger groups of disks.

HP does have a sanctioned approach to do this type of operation. It's called Split/Recombine Mirror Array.

Edit:

You're doing this with VMware ESXi. It's unnecessary to go through this trouble for ESXi. When you upgrade VMware, there's an option to revert to an alternate Bootbank.

See: How is the /altbootbank/ partition used in ESXi?

Or the VMware ESXi architecture paper.

The ESXi system has two independent banks of memory, each of which stores a full system image, as a fail-safe for applying updates. When you upgrade the system, the new version is loaded into the inactive bank of memory, and the system is set to use the updated bank when it reboots. If any problem is detected during the boot process, the system automatically boots from the previously used bank of memory. You can also intervene manually at boot time to choose which image to use for that boot, so you can back out of an update if necessary.