I’m considering building a media player and using the PiDrive enclosure, but would like more than 1 TB of storage.
If I purchase the piDrive power supply and cable etc seperately, could I use a standard WD internal HDD with it in order to increase capacity?
You’d need a suitable enclosure or adapter cable to convert the drive’s connector (SATA or IDE etc) to USB, as a standard HDD won’t have that connection. The adaptor cables aren’t expensive, but would be something to remember to get.
The PiDrive has that built-in, which is part of its selling point. All you need do is connect a suitable USB-microUSB cable to it and off you go.
You could go this way, but just as @DarrenHill explained you’ll need some type of convertor (enclosure or adapter), however note that if you decide to go for a 3.5" drive you should have an enclosure or a SATA to USB adapter with an external power source since you won’t be able to power it up via the USB ports alone.
The currently offered PiDrive enclosures are meant to work with a specific setup and I don’t think you’ll be able to fit a 2.5" drive with a SATA to USB adapter in there, but you can always try it out if you want.
As for a 3.5" drive, it will definitely not fit in any of the PiDrive enclosures and I was talking about an external enclosure for the drive itself, so that you can use it with a USB interface and connect it just as any other external HDD.
I can second @JoeySmyth 's experience. I routinely use a MyPassport 4TB with a Pi Zero and Kodi (LibreElec) and it works beautifully. That is using a powered hub board connected directly to the zero, and it’s more than enough to power everything (the load is very borderline for a direct connection, but a powered set-up works).
It also works fine via a PiDrive cable, as he says (using an adaptor for USB to micro-USB for the input of course).