Database backup taking longer than usual to a networkshare The SQL server does backups with this command to a network share \BackupServer\weekeklybackups"

BACKUP DATABASE DBNAME TO DISK = '\\BackupServer\weekeklybackups\DBNAME.BAK'


There is a 1 gig network between the SQL Databse server and the BackupServer. but it uses maximum 25% of the network, so the backup is taking for ever.
If I make the database locally and copy it over to the same network share, it is faster using 99% of the network bandwidh.

So, the weekly automated backups are taking a long time, which is an issues

When I check the monitor it says it is waiting with "ASYNC_IO_...". When I did some google search it says, this is an issue on tape backup. But this is on iSCSI hard 

disks. and manual copy over works fine. So, what could be the issue? It was working fine for a year until last week. Not sure what happened. but where should I be 

looking in to?

Thanks for your time
It was an issues on the hard drive. It never showed any errors in event viewer or the controller. When we rebooted the server, the errors showed up and we replaced the drives.