I used hdd for system in Nas4free and this hdd is now system disk for Ubuntu server, but some zfs information is still there...I didnt wipe disk before installing new system. Can I remove this metadata now or I must wipe complete disk and install Ubuntu server again?
It's an old question, but just for the records:
The metadata of zfs is stored in the first and last two 256kB of each disk involved in zfs.
So you can do things like this:
sudo dd if=/dev/zero of=/dev/ada1 count=1 bs=512k
for the first bits and after you found the number of all sectors of your device, you can do this for the last metadata bits:
Assuming a sectorsize of 512 bytes, we have
262144 bytes (256kB of metadata) / 512 bytes (sectorsize) = 1024 sectors
sudo dd if=/dev/zero of=/dev/ada1 oseek=NUM_OF_SECTORS-1024
now, you should see a clean device.
Note* - /dev/ada1 is a device in freeBSD. Mounted in Linux it might be /dev/sdb, /dev/hdb etc.