A system disk runs the server operating system. It is like drive C in a PC.

Data disks store the server data. They are like drive D, drive E, and drive F in a PC.

Yes, you will be billed by disk capacity right after you have purchased the disk, regardless of whether the disk is attached, all disk spaces have been used, or the server is stopped.

Data migration involves the following scenarios:

Cross-AZ data migration: Disk data can be migrated from one AZ to another through disk backups. You can create backups for your disks using the CBR service, and then use these backups to create new disks in the target AZ. For details, see Creating a Cloud Disk Backup and Using a Backup to Create a Disk.

Cross-region data migration: You can create a data disk image from the data disk in the current region and replicate the image to the other region. Then you can use the data disk image to create data disks in that other region. For details, see Creating a Data Disk Image from an ECS.

Cross-account data migration: You can create a data disk image from the data disk under one account and then share the image with another account. Then you can use the data disk image under that second account to create data disks. For details, see Creating a Data Disk Image from an ECS.

A shared disk can be attached to up to 16 servers.

EVS disks that have been attached to servers can be backed up along with the servers using CBR. In addition, CBR cloud server backup allows you to replicate backups to a different region and create images in that region using replicated backups.

EVS disks that have not been attached do not support cross-region backup and restoration.