I had (have) been running OMV for a long time.
I upgraded ages ago by pulling my data drives , which are three disks in a md raid 5, installing OMV again, then putting the data disks back in. OMV then detected the RAID and LVM, and it worked.
So this time, going from 0.5 to 2.1
Pulled the data disks, installed 2.1
I now find that the md assembles as md127 showing UUU on raid 5:
md127 : active (auto-read-only) raid5 sdd1[0] sda[2] sdb[3]
3905399104 blocks super 1.2 level 5, 32k chunk, algorithm 2 [3/3] [UUU]
But, it won't find the LVM PV that I had on the MD.
pvscan returns:
/dev/md127: size is 7810798208 sectors
/dev/md127: size is 7810798208 sectors
/dev/md127: No label detected
There's no partition table showing on /dev/md127, but I honestly can't remember if I partitioned the md raid device before running pvcreate on it initially.
I can dump some of the md127 using dd, so it's readable and it looks like I get some sensible data.
Hoping it's some LVM metadata incompatiblity between 0.5 and 2.1, is this possible?
I'd love some answer as to how I can get LVM running on this again. 
Thank you