sql server backups - what is physical_block_size?

we have recently changed the file server that is the repository (the place where the backups are stored) server . I noticed the hardware specially IO is much better backup take shorter to finish, depending on many things, however, restores still take the same time as before. this is not a complex question, I have only one question here: what is physical_block_size? when you run the following query on the msdb database: SELECT * FROM msdb.dbo.backupmediafamily BMF WITH(NOLOCK) order by 1 desc this is the result of above query and please notice how the physical_block_size changed from 4096 to 512 exactly when I decided to use our new server mentioned above for the destination of the backups: