I bought a seagate 14TB external HDD and have added several TB of files to it. Its running slow so I wanted to defragment it.
When I right click on the hard drive, click properties, click tools, there is an option to defragment the hard drive.
However it is grayed out for the external HDD. However there is still an option to defragment the internal SSD on my laptop.
Any idea what that means, and if I can still defragment the external drive?
Download the Seagate utility SeaTools and try it with that. Health 
Fix All Long
There are unfortunately several reasons you could have trouble. The main two I can think of that you can fix are that the drive may need to be scanned (which I think is in the same place the defragment option is in properties), or that it might not have enough space ( you need, IIRC, about 15%)
If it’s not an NTFS drive, then Windows can’t defrag it, but third party tools can.
Option 3: it’s actually a Solid-State Disk and you never defrag those. Every Windows disk maintenance utility knows and honors this.
It shouldn’t be. Its 14TB large, and makes noise. Also the price was about $15/TB, which is far lower than the price/TB of SSDs which runs closer to $60/TB
Yes, but it will still appear, and will let you “Optimize” the drive. Instead of defragging, it will TRIM the drive.
That’s reasonable, given both the pricing and the physical attributes.
I can’t picture a non-NTFS 14 TB drive, to consider @BigT’s speculation.
How much stuff have you ever deleted from the disk since it was new? If you pretty much have done nothing but added files over time, and not edited any of those files later, nor deleted any, well, the drive has zero fragmentation. Windows may well be seeing that and disabling the option for that reason.
Did you get the drive from a reputable dealer? 14TB sounds like a good size to attract scammers who fake a larger size. They tend to also format the drive to exFAT instead of ntfs to make it harder to use Windows tools on it.
Drive sold as 16 terabyte = 14.55 tebibytes, and subtract a little more for unusable space, and it’s reasonable OP has about 14 to use.