I have a 3TB Seagate. The disk was from an raid system and had 1 partition sdb1 with 3TB and ext4 filesystem. As i wanted to prepair a new disk for a raid, i exchanged in sgdisk sdb1 with an raid disk which had sdc1 sdc2 sdc3 sdc4. After that mistake i take another seagate sdd with same size 3TB and ext4 and do an sgdisk over sdc. That is the actual status of sdb. I have make a backup with ddrescue to another disk and tried testdisk and parted, but could repair the partition table.
Now i need to do it right, because i am not sure which commands in testdisk or sgdisk are right.