External SSD Samsung T5 doesn't show up anymore After I had got a new Samdung T5 external SSD (2 TB) I had formatted it with DiskUtilit, created 3 partitions and could make it work:
1. TimeMachine Backup to partition 1
2. Some important data backup in partition 2
3. Start up with macOS Mojave in partition 3

The partition which came with the SSD (Setup utility) was untouched.

After my MacBookAir (early 2015) ran out of power and went to sleep mode,  restarted the computer.
Since then only the partition with Setup utility is visible. I am not able to mount any other partition nor can I see them in the diskutil in Terminal mode.

Can you help me? 
What makes me frustrated is that the SSD was working fine for several days, but now all data seems to be lost or unaccessible.
Probably a format issue. There should be nothing left that was on the disk when new!
Format the drive completely in Mac OS Extended (journaled), completely, everything!
Then partition it in the three partitions.
Then install Mojave in one partition (the format will be changed only in that partition).