If you have only two drives in a storage pool in Storage Spaces, and you disconnect the drives from your PC and don't reconnect them at the same time, the drive that isn't connected might become separated from the pool. Your storage space can't use the separated drive to help protect you from drive failure until you reset the drive to reunite it with the pool.
If you added or changed files on the drive while it was separated, they'll be permanently deleted when you reset the drive. To review the files before you reset the drive, follow these steps:
Disconnect both drives.
Connect the separated drive.
Use File Explorer to view the files on the drive, and then copy any files that you want to keep to another drive.
Disconnect the separated drive.
Reconnect the other drive, and then reconnect the separated drive.
In the Storage Spaces Control Panel, under Physical drives, tap or click Reset next to the separated drive, and then tap or click Reset drive.
For more info about Storage Spaces, see Storage Spaces: FAQ