[Tolerance]

Alright, so I'm a mid level diamond Protoss having trouble with blue flame hellion drop.
Especially in the map Typhon Peaks, where Terran can drop in the bush and I cannot see it until its too late. gateway units are just too slow, everytime I killed all the hellions, I lose like 15 probes, sometimes when I just expanded blue hellion can come from multiple locations, with forcefield I really cant stop them once they get into my main, because they are so fast, and the amount of stalkers I have cannot kill them or outrun hellions. Anyone knows how to counter this?
I have already scouted it, just that I'm caught off guard in maps like Metalopolis, Tel'Darim, Typhon Peaks etc.
Even if I can stop hellion before they get to mineral line, terran always expand with bunkers set up and I cannot really do anything until I have enough units to counter all hellion drops or immortal to push through bunkers, im scared to take expansion in some maps because they have a wide range of space in front of natural for hellions to run around with like Xel'Naga and Slag Pits.
Even if I spilt up my probes, half my probes die and I will lose so much more. Any counter to this?
It happens at around 7 min mark usually when I'm expanding.

[AxSGRiM]

Get used to leaving stalkers in your base. Observers should tell you when they drop.

[ROOT`iaguz]

Few tricks.

First up, do you have tower control? If you have control of a tower(s) that give you vision of the path between your bases then you can keep your units near a mineral line in case of a drop or banshees. You won't get attacked in your front becasue you'll see it coming.

Second, place a few pylons around your perimeter and keep a close eye on the minimap for the telltale red dot (turn allied colours on so said dot appears very obviously.

Other then that there's not a lot you can do other then hope you have your units near your mineral lines in the right way when the drop hits.

[SkyBreaker]

I'm not a diamond level player, nor a Terran player but from what I've read from SC2SEA's TGM; good scouting (as Iaguz outlined) combined with a good sim-city (i.e. building structures around your mineral line to prevent easy access to your probes) is a sure-fire way to be secure from hellion-harass, or at the very least minimise the amount of damage you will receive from the harassment.

[Nemo]

Scout, Simcity, all excellent advices given above.

There is something else. 4 Hellions + Blue flame research are expansive and especially at the 7th minute mark. Just after his first drop you should be able to kill him right away if you have made enough units. If not then you're on 2 bases, then even if you lost probes, you're still in the game because he just begun (at best) his own.

That's one of my favorite openings against Toss (iEchoic 3 Helions drop followed by banshees). It's very vulnerable to early pressure and to FE turtling Toss.

[AvPoker]

1. You need to know when the terran has those starports up and running so you know the approximate time of when he'll be dropping your base with an observer.
2. Around that time, you should have a habit of planting pylons everywhere, such as next to cliffs or at corners, so your more likely to see the drop coming.
3. Once a threat of a drop arrives, you need to keep 5-6 stalkers at your main to intercept drops until you can afford cannons + high templars/dts off 3 or more bases while leaving the rest of your army parked around the natural.