A short backstory: something happened on my Synology NAS, apparently a buggy mailserver program that came with an update stopped the device from seeing the 2x3TB drives I have. After a lot of searching and a trying a lot of different solutions I eventually got the drives to show up again but they would not mount. After struggling, I figured that, because it was set up as a mirror, I could format one drive, mount it and perform the recovery on the other.
EDIT: I had it wrong and was trying to mount the wrong disk (not used to messing around with raid), however, I'm still unable to mount the correct disk,
$ file -s /dev/sda1 /dev/sda1: data
$ mount -t ext4 /dev/sda1 /mnt mount: mounting /dev/sda1 on /mnt failed: Invalid argument
$ cat /proc/mounts rootfs / rootfs rw 0 0 /dev/root / ext4 rw,relatime,barrier=0,journal_checksum,data=ordered 0 0 /proc /proc proc rw,relatime 0 0 /tmp /tmp tmpfs rw,relatime 0 0 none /dev/pts devpts rw,relatime,gid=4,mode=620 0 0 /sys /sys sysfs rw,relatime 0 0 /proc/bus/usb /proc/bus/usb usbfs rw,relatime 0 0 /dev/vg1000/lv /volume1 ext4 rw,relatime,synoacl,barrier=0,journal_checksum,data=writeback,jqfmt=vfsv0,usrjquota=aquota.user,grpjquota=aquota.group 0 0 /dev/vg1000/lv /opt ext4 rw,relatime,synoacl,barrier=0,journal_checksum,data=writeback,jqfmt=vfsv0,usrjquota=aquota.user,grpjquota=aquota.group 0 0