I've done around 10 flights now, and recorded video from about half. After I download the footage to my computer, I delete the video files off the micro SD card, but it doesn't free up any space on the card. I'm down to 3GB left on the 16GB card.
What am I doing wrong?

What happens if you format the card (can you do this in the drone?) does this free up the space?  Normally all I do (in my P2) is what you did, just delete the actual video files and leave everything else there and it worked fine.  I also do this for my other cameras (GoPro, etc.)

Try this: http://www.wdc.com/en/products/products.aspx?id=1330

It has SD card reader and it has its own battery power, so save your files off the card on the spot so you can keep flying.

I keep the 16GB card on the shelf for updating firmware; carry (2) 64GB 95Mb/sec cards when out flying around.

Memory is very inexpensive

keep calm Posted at 2015-5-15 06:29
Try this: http://www.wdc.com/en/products/products.aspx?id=1330

It has SD card reader and it has its ...

This looks interesting, but still doesn't solve my storage issue on the card itself...

john.lambert4@o Posted at 2015-5-15 06:22
What happens if you format the card (can you do this in the drone?) does this free up the space?  No ...

I haven't tried formatting the card. I'm afraid it's going to guttered of everything - including the firmware update that DJI recommends you keep on the card.
There has to be a better way! I dont want to have to re-update my firmware every time I format my card for space. I've never had this issue with any of my other cards - GoPro, DSLR footage. I've always just imported, and then deleted the files.

knicholas194@gm Posted at 2015-5-15 07:28
I haven't tried formatting the card. I'm afraid it's going to guttered of everything - including th ...

I don't think it was DJI's intention for you to keep the firmware update on your memory card after an update.  I formatted my card a few times since my last firmware update and have had many flights without a hitch.

The only reason they suggest keeping the firmware on the card is that it also updates the battery. So if you put a new battery into your quad, you will need to have the file there to update the firmware of the new battery. Otherwise, there is no reason to actually keep it on the card. Are you using an external card reader or plugging the quad directly into the computer? It is POSSIBLE that if you are plugging in the quad that it might write-protect it so you can copy from the card but not actually delete. Formatting is the best option in any case as a formatted card is always fastest.

Not to be condescending but I can't believe this thread is this long without an easy answer. Here is the reason - put the card in the reader and then empty your wastebasket. Done and done. Really? Maybe you need a new card. Really? Make sure you format it so you lose everything on the card. Really? Sorry, couldn't help myself. I'm not even a tech guy and this is so obvious. This just shows you that people are trying to help but they really don't know the answer. Welcome to Internet forums.Haha

Before a flight, when you power up the quad and open the app, a status screen is displayed. At the bottom of this status screen you have the SD card available space and a button to format the card. As simple as that !
Should be part of your checklist !
And to avoid any space/format problem, the best to do is to always format in the device (the same for cameras or anything else)

Cacc Posted at 2015-5-15 13:04
The only reason they suggest keeping the firmware on the card is that it also updates the battery. S ...

No, I'm not plugging in the quad, I am using a card reader on my mac.

BrewserB Posted at 2015-5-15 14:32
Not to be condescending but I can't believe this thread is this long without an easy answer. Here is ...

Before you start making fun of people in this thread, that was the first thing I tried. I emptied out all the contents of the card, But it didn't open up any space on the card. It still shows around 3.5GB free out of 16.
I'm not a noob with SD cards, I just can't understand why this isn't working like all the other cards I use.

Ulysse Posted at 2015-5-15 14:42
Before a flight, when you power up the quad and open the app, a status screen is displayed. At the b ...

I will try this, the next time I fire up the quad, Thanks!

knicholas194@gm Posted at 2015-5-16 01:38
Before you start making fun of people in this thread, that was the first thing I tried. I emptied  ...

With the Mac, if you delete files from the SD card they will just be hidden unless you also "Empty Trash".  I ran into the same issues a while back with my GoPro.  

Ulysse Posted at 2015-5-15 14:42
Before a flight, when you power up the quad and open the app, a status screen is displayed. At the b ...

I formatted the card in the app, and it worked, back to 16GB!
This will now be a part of my pre-flight checklist.
Thanks!

If you delete the files in the reader hooked up to your computer, dump the trash before you remove it, otherwise the files remain on the card, but may be invisible.

knicholas194@gm Posted at 2015-5-16 01:38
Before you start making fun of people in this thread, that was the first thing I tried. I emptied  ...

Don't let that arse clown get you down. Internet tough guys/trolls are always frowning. Let them live in misery. Stay up Fly safe. Hey Clowns, Stop being D nozzles. If you don't like someone elses question or can't or won't help them then leave them alone.  

Always format thru the app.  You can do it on the computer (don't forget to empty the trash) but then reformat in the app/camera.  That way it is formatted to the use of the drone.  Each camera has it's own internal format design.

Did you actually dump the trash after you deleted it on subject computer?

I was having similar problems with trying to free up storage on my Mac.  I purchased a $10 app called Daisy Disk.  Scans your HD and any external storage for all the hidden files that are eating away at your memory so you can delete them.  

Yes, you have to dump your trash on the mac or else it will revert back to the card once you eject.  But as I
said I reformat in the camera through the app.

knicholas194@gm Posted at 2015-5-15 07:28
I haven't tried formatting the card. I'm afraid it's going to guttered of everything - including th ...

the only reason to keep fw on sd card is to update your batteries, which im sure you have already, the fw you can always dump back on sd card if u ever need to.
as far as why u cant delete; what os do u have?

btw...
Super Empty Trash (osx)
is free and gives you the opportunity to empty the trash on different volumes separately
http://faqintosh.com/risorse/en/sys/setrash/

I found this thread looking for a fix for the same issue on my OSMO. It is the same solution (format SD card), just a little harder to find. For OSMO, go to settings (upper right) -> Camera-> Format SD (near bottom).

Thanks for your help!

I don't have this problem because I have disabled the trash can on my windows machines.  In over 30 years of computing, I have never regretted doing that.  If I accidently delete something,  I immediately run a file recovery program.  I very rarely need to do that.  When I delete something, I want it gone. I don't want to have to delete it, say yes, I really want to delete it, then have to empty the trash to delete it again later.  I don't need my hand held like that.  The same way,  when I press the off switch on my phone, I resent having to then confirm that I want to turn it off,  then say okay again to actually turn it off.  I don't do this with my car,  or my motorcycle or even my tv or the lights in my house.  It is maddening.

That said and rant being over,  here are a couple of points:
Leaving the bin file from the fw update on the card does unnecessarily take up space,  though not much.  But it also forces the Phantom to examine that file and compare it with its fw every time you start the Phantom,  which makes it take longer to be ready to fly.  If you don't need it,  delete it.
Formatting the card after or before every flight ensures maximum space available,  keeps the card properly aligned and ready to operate at its max potential without having problems like stuttering recording or playback.  Formatting also is a good way of quickly ensuring that the card is working.  If it can't format, time for a new card.  Formatting in the Phantom, I mean.  As to where to format,  in the Phantom or in the Computer,  I have seen arguments for both from even DJI employees.  Personally,  I format in the Phantom normally,  and every now and again I do a complete format it the computer to ferret out bad sectors,  keep the non existant cylinders aligned, ( a holdover from the days of actual hard and floppy disks, but does not hurt and may help ) and another reason that escapes me right at the moment.