Heyo,
I put together a few videos showing EVERY location of each lost sector area across all 4 planets. Some are very well hidden so If you can't find them you can always reference one of the videos. Doing these nets you your characters missing subclasses and can level you really quick. I level 20 well before the ending of the game.
[b][u]EDZ[/u][/b]
Atrium - https://www.youtube.com/watch?v=AANYrNQaUIs
Widows Walk - https://www.youtube.com/watch?v=vaUtnMRsc0M
Terminus East - https://www.youtube.com/watch?v=Xk8uGhC_8LM
Scavenger's Den - https://www.youtube.com/watch?v=R2WLFERLcIo
The Drain - https://www.youtube.com/watch?v=N3cYo6Iddz0
Whispered Falls - https://www.youtube.com/watch?v=peoLGAr-rCY
The Weep - https://www.youtube.com/watch?v=lhdbKgA-YD8
Excavation Site XII - https://youtu.be/uUMP3wXGQdc
Flooded Chasm - https://youtu.be/xyeCzoSA4co
Hallowed Grave - https://youtu.be/6ImgyV9LaDg
Pathfinder's Crash - https://youtu.be/Fzv0q1Ovzno
Shaft 13 - https://youtu.be/Hk_qPO_DuM8
The Pit - https://youtu.be/SzcZ92lxwOU
Skydock IV - https://youtu.be/9Rnt0MI4k5o
The Quarry - https://youtu.be/lJ6jTRZj89g
[b][u]Titan[/u][/b]
Cargo Bay 3 - https://www.youtube.com/watch?v=DI3KainXCC0
Ds Quarters-2 - https://www.youtube.com/watch?v=A7JZBmMZzsQ
Methane Flush - https://www.youtube.com/watch?v=Xgfy9O8WzK0
[b][u]Nessus[/u][/b]
Ancient's Haunt - https://www.youtube.com/watch?v=DOJFoseN380
The Carrion Pit - https://www.youtube.com/watch?v=4IqLRVeOBB8
The Conflux - https://www.youtube.com/watch?v=nvUUnFHopGo
The Rift - https://www.youtube.com/watch?v=0nxg7-9vGVM
The Orrey - https://www.youtube.com/watch?v=2p5DiPV7kl4
[u][b]Io[/b][/u]
Sanctum of Bones - https://www.youtube.com/watch?v=KS8MMXJls1g
Aphix Conduit - https://www.youtube.com/watch?v=LiUr1UnlBsg
Grove of Ulan-Tan - https://www.youtube.com/watch?v=5QqCAZbmcJU
Hopefully this was able to help you!
Edit: Will just point out that these places are farmable for gear and tokens.
Edit 2: If you find that you don't need to use these then don't, that is perfectly fine. They are more a reference to those who don't have to time or patience to hunt down entrances to these areas. Please have respect for BOTH sides of the playerbase, not everyone plays the way the other person does.
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Later :)