I was trying to mount /dev/sdi on /mnt/mail2 and by mistake I pressed enter too early and mounted on /mnt/mail, that was already mounted with /dev/sdd.
# mount -o noatime /dev/sdi /mnt/mail
To my surprise, doing df, showed both partitions mounted.
# df -h Filesystem Size Used Avail Use% Mounted on ... /dev/sdd 163G 280M 154G 1% /mnt/mail /dev/sdi 163G 280M 154G 1% /mnt/mail
With umount it removed first /dev/sdi and the second time /dev/sdd.
One thing to notice is that the space shown by df is actually the one of the new sdi disk, while when mounting sdd alone df says:
/dev/sdd 148G 148G 0 100% /mnt/mail
Any reason for this strange behaviour? Could it trash my data in case I didn't notice at once and did some write?