Do you have physical access to the machine? If not, I'd first replace sdb and low-level-copy sda's content using ddrescue to sdb into an image file. This file can be mounted and you can run fsck on it. Maybe this will bring back most of the data.
Question: what type of hdd crashed? Last year, within three weeks, three HDDs crashed in my server. All of them were Seagate 3TB SATA.
In general: raid1 doesn't replace a working backup!