I was able to work around this by using the command line to create a disk on the datastore and then adding it as an existing disk to the VM. I’ve described the problem and workaround more thoroughly in my blog post: How to Add a Hard Disk to an ESXi Virtual Machine, but I just wanted to report it since I do believe this behavior is unintended.

Hi everyone,

I recently deployed a new ESXi host (running HPE Customized Image ESXi 6.7.0 Update 3 version 670.U3.10.9.0 for HPE Servers based on VMware ESXi 6.7.0 Update 3) to my homelab and I believe I discovered a bug in the ESXi web client. When you try to add a hard disk (new standard hard disk) to the VM via the web client, if the default datastore is near capacity, the save button grays out. That part is expected, but what isn’t expected is that when you switch the disk to a datastore that has room, the save button stays grayed-out never to re-enable. Heck, even if you delete the new disk altogether, the button stays grayed out. You have to close the entire edit modal and reopen it to regain the ability to save. It’s as if the web client only evaluates the conditions once and, once it fails that initial check, disables the save button permanently, never to re-evaluate the save conditions again.

Anyway, to work around this – after selecting a datastore with free disk space – either toggle thick/thin provisioning, or temporarily specify a thick disk size that’s larger than the free disk space (e.g. make GB -> TB). One of these options should enable the Save button again. Then configure the settings as needed, and save the settings.