Well, here’s what was the problem … PEBKAC (Problem Exists Between Keyboard And Chair).
If one asks you to try to recover some data from a supposedly RAID1 array and you see an unknown file system … stop wasting your time figuring out what exotic file system could that be … instead, browse that perfectly mountable ext3 partition, where /etc folder resides and have a look at mdadm.conf file. You know, just to check if the device wasn’t configured by a person dump enough to use “linear” instead of a mirror. The device I was looking at was.
DEVICE partitions
ARRAY /dev/md1 level=raid1 num-devices=2 UUID=448b6d48:e61b8c2c:d8164f57:15f1a403
ARRAY /dev/md2 level=raid1 num-devices=2 UUID=e82637b4:1e4b8b82:04b60160:9e819908
ARRAY /dev/md3 level=raid1 num-devices=2 UUID=a433ab64:9077fd6c:b80fa81e:e364f901
ARRAY /dev/md4 level=linear num-devices=2 UUID=4e119873:258a8cf4:1688b5b1:b508a7a4
Then you can tell the happy owner of the device that, sadly, he/she needs both two disks functional. Well, at least the swap partition is mirrored. 
1.5TB does sound better than 750GB, but, well, without mirroring. 
