This is my first time using Freebsd, ever. I have installed RPI3 freebsd 12.0 release on SSD card 15gb, no patches at this stage. My plan at this stage is to install the AMP part on the external hard drive which has been mounted and partitioned (30gb) for this to occur. Now I have read that freebsd using pkg i.e:
pkg install apache2.x

However, I am not sure how to point the package install to the external hard drive and whether or not this works? Having gone through the steps for creating, partitioning and add the user to group I can see that freebsd is more involved then I am used to yet, I am willing to learn. So my questions are:

1. Can I set up AMP on external hard drive and point it to the Raspberry PI and vice versa?
2. If so, what are the steps or appropriate documentation to do this?
When you install third-party software via packages or ports, them get installed inside of /usr/local, just mount your external hard drive in /usr/local then install with pkg

I don't understand the point 1, you need to clarify.
Ah huh, I read a post where the person moved the user files to the next hard drive, but your idea seems easier to implement. Thank you.

For question one, I was referring to the concept I was reading in freebsd handbook about usb-disks, in particular this line where the author sets a localrule in
add path 'da*' mode 0660 group operator
in /etc/devfs.rules and then points to localrule in the file /etc/rc.conf via

I just wants sure if I needed to do something like this to implement what I was to do, again, I am very new and trying to cover all bases before I install.
Is there a specific reason for wanting the packages on a separate disk? You could conceivably put /usr/local, or even /usr (which is often a separate partition anyway) on an second disk, although ideally you'd want to do it at install time as there's likely already files in there that will need to be moved.

I have a server running Apache/PHP/etc a dozen or so websites. That's using 13GB of space in total, 7GB of which is website files. Storing the website files (and MySQL data if needed) on a standalone disk would be a lot more straight forward than trying to actually install the applications directly onto it.
my apologies, yes it is a SD card, and I have tried that with Rasbian operating system however it wouldn't load login page. Raspberry pi's seem to require a certain amount of the system to be mounted via sd or ubs card.

The only problem, usdmatt, is that I will need to install rstudio server which has a lot of libraries and ran out of room using Rabian and kept getting an error when mounting the external hard drive in fstab, had the same problem with freebsd so I'm just not mounting it in fstab. Sort of a solution :) But I get what you are saying with the files which seems to make sense.
Hmm, seems a bit odd to run a fairly heavy graphical IDE on a RPI to me... I was assuming a headless FAMP server.

If you want to run a GUI and heavy applications on the RPI then maybe the most straight forward solution would be to mount /usr (and possibly /var as that gets a fair number of writes) on the second disk during install so that everything other than the core system is on it. Probably not that difficult to mount these manually and customise the fstab during install, but it's not something I've done myself so wouldn't be able to give any clear instructions without actually trying it.

I'm not that keen on a big chunk of the system being on an external disk though. You'll probably get a fair number of errors if the system tries to run and the disk isn't present for some reason.
usdmatt, yeah it is probably a bad idea, I have had nothing but trouble and now it hanging. Thanks for the replies. It is a lot easier just making it a file server which was my back up plan.