Recently changed my lvm data to include another disk, and rebooted. Linux fsck'd the disk on reboot, now when using parted get

"Error: /dev/mapper/pve-data: unrecognised disk label"

The drive is still mounted, and I can read/write the data:

/dev/mapper/pve-data on /var/lib/vz type ext3 (rw)

I am currently backing up all the data on the drive, just in case... is there anything I can do to fix this without purging the data on the drive?

parted is a partitioning tool - it should be used on the underlying disks NOT on the LVM Volume Group VG or logical volume (LV) mapped devices. That is to say typically you're running parted against things like /dev/sda, /dev/sdb etc... NOT /dev/mappper/... items. From your df it appears that /dev/mapper/pve-data is one of your LVs (presumably the LV is "data" and the VG is "pve".

If you run "vgdisplay -v" on your VG what does it show for LVs? PVs?

If for example you saw /dev/sdb1 as a PV in your VG then that is the first partition on your second disk. You'd use parted on /dev/sdb.

However, once you've added disks as PVs into a VG you shouldn't be doing anything with parted or other partitioning tools unless you're adding a new disk and need to partition it.