Creating new zfs pool mixing previously used drives

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I am building a new homelab for myself using Intel NUCs. I need a NAS so I can test live migration and I was set to buy 2 bay synology. But when a 140$ HP Gen8 Microserver was on offer I couldn’t say no. FreeNAS might not be as polished as Synology but I would be getting 4 bays, two gigabit ports and iLO at a third of the price. Also it would offer the possibility of learning FreeNAS (my other, longtime NAS is slackware + zfs on linux).
Since this server is not going to hold any important data (I’ll be sure to backup anything important on my production NAS – i.e. the one I don’t mess with) I wanted to reuse three old 2TB drives left from upgrading my main NAS to 3TB reds.
When creating the volume in FreeNAS I would be getting the following error:
Creating GPT partition scheme on ada2gpart: geom ‘ada2’: File exists
I imediately realized that this is because some of the disks used to be in a zfs pool in the past. The suggestion was to clear the begging of the disk but that didn’t help. The only fixe from the ones proposed here was:
# destroy ALL DATA on disk /dev/ada2
sysctl kern.geom.debugflags=0x10
dd if=/dev/zero of=/dev/ada2 bs=1m
# now reboot!
I don’t remember if I zeroed the entire 2TB disk or just left it fot ~10 mins to ensure most of the zfs metadata is gone.
After running the above in the command line I was able to create the volume just fine.