NTbackup failes to backup 2000 mailboxes

When I try to backup the exchange 2000 server using the NtBackup that comes with Win2K the error “you do not have permission to access portions of M:\ & M:\Domain_name & M:\Domain_name\MBX ” the administrator account is a member of the domain admins\Enterprise admins account which in turn is a member of the backup operators security group. I jhave checekd that the administrator has full access to both hard drives So what is wrong ?????????????

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