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1.    /dev/sda it's 100% your internal drive. Your external drive may be sdb, sdc or another one.

o   The disk names in Linux are alphabetical. /dev/sda is the first hard drive (the primary master)

o   Root device volume contains the image used to boot the instance.

2.    Amazon EC2 instance store: which means the root device for an instance launched from the AMI is an instance store volume created from a template stored in Amazon S3. 

3.    Backed by Amazon EBS: This means that the root device for an instance launched from the AMI is an Amazon EBS volume created from an Amazon EBS snapshot.

4.    We recommend that you use AMIs backed by Amazon EBS, because they launch faster and use persistent storage.

5.    Instances that use instance stores for the root device automatically have one or more instance store volumes available, with one volume serving as the root device volume.

6.    When an instance is launched, the image that is used to boot the instance is copied to the root volume. Note that you can optionally use additional instance store volumes, depending on the instance type.

7.    Any data on the instance store volumes persists as long as the instance is running, but this data is deleted when the instance is terminated

8.    Instance store-backed instances do not support the Stop action

9.    Instance Store (ephemeral storage): Temporary block level storage. It is physically attached to your instance.

10. Instance stores cost is part of EC2 instance only.

11. If you plan to use Amazon EC2 instance store-backed instances, we highly recommend that you distribute the data on your instance stores across multiple Availability Zones.

12. You should also back up critical data on your instance store volumes to persistent storage on a regular basis.

13. Instances that use Amazon EBS for the root device automatically have an Amazon EBS volume attached.

14. When you launch an Amazon EBS-backed instance, we create an Amazon EBS volume for each Amazon EBS snapshot referenced by the AMI you use. 

15. An Amazon EBS-backed instance can be stopped and later restarted without affecting data stored in the attached volumes.

16. There are various instance and volume-related tasks you can do when an Amazon EBS-backed instance is in a stopped state. For example, you can modify the properties of the instance, you can change the size of your instance or update the kernel it is using, or you can attach your root volume to a different running instance for debugging or any other purpose.

17. If an Amazon EBS-backed instance fails, you can restore your session 

18. By default, the root device volume for an AMI backed by Amazon EBS is deleted when the instance terminates.

19. Use the modify-instance-attribute command to preserve the root volume by including a block device mapping that sets its DeleteOnTermination attribute to false. (Hence, you can change this parameter event instance is running).