Change the words “attach block storage” to “replace with block storage” Attach means “add on to”, implying that the storage amount you are selecting is in addition to the amount you already have. It is not clear at all that you need to “Attach” your existing storage as well as any extra you require.

Prevent the process from starting if the requested amount of storage is less than the amount currently in use on the server -This is a bit of a no-brainer. If my site is using 100gb of storage, I shouldn’t be able to select “attach(ie replace) my current storage with 30GB” - which is 70GB less than is already in use. It's a simple if x is great then y, disallow.