AWS – Why Can’t Attach New Volume to Existing Instance

They just released RI Expiration alerts today! They're in the top right-hand corner of the RI Summary page in Cost Explorer: https://aws.amazon.com/about-aws/whats-new/2019/05/reservation-expiration-alerts-now-available-in-aws-cost-explorer/

Starting today, you can use AWS Cost Explorer’s new reservation expiration alerts capabilities to receive email alerts 60, 30, or 7 days in advance of your reservation expiration date, as well as on the date of expiration. These notifications can be sent to up to 10 email recipients.

To turn on reservation expiration alerts, navigate to the Reservation Summary page, locate the reservation expiration Key Performance Indicator (KPI) in the top right corner, and click on the “Manage alerts” link. From there, simply indicate when you would like to receive reservation expiration alerts, and AWS will begin monitoring your reservation portfolio and automatically send you alerts.

All instances are created from an Amazon Machine Image (AMI). The volume or volumes attached to a new instance are volumes created from the snapshots linked to the AMI, and that size determines the default size. If your instances have 8GB root volumes, that's the size of the root volume snapshot of the AMI.

When creating an instance you can, of course, specify larger (but not smaller) sizes for the volume(s), but that default isn't a "preference" that you can customize -- it's an attribute of the AMI you are using to launch.

You can, of course, create one instance from one of the stock AMIs, but with the volume size you want, then make a new, private, custom AMI from your new instance... and when you launch new machines using your custom AMI, the volumes will automatically be the size you wanted.

So, yes, you can get the behavior you want, but it's done using a mechanism other than what you had in mind.