You can use ANY type of storage device as an external device through the USB wether it is a small flash drive,SD card,M2 card,HDD,SSD and so on...
They will all work just fine.
As for the data transfer speed...well...that really depends on the type of your ports on the motherboard.For example...if you use USB 1.1 and connect SSD to that port to transfer data,the speeds are not gonna be great even if you have the best SSD drive since USB 1.1 cannot provide so big data transfer speed.In these kind of cases this is usually the main reason why people use HDD as an external storage device instead of SSD.
But will they BOTH (HDD and SSD) work on ANY USB port (1.0,1.1,2.0,3.0?
Answer: YES (unless if your USB port is bad...in that case everything will work like crap or it won't work at all lol
)
Same applies for all your SATA ports (if you have any).
Answer: YES
So if super high data transfer speed is SO IMPORTANT TO YOU for an external storage then you should definetely use SSD on a great SATA,eSATA ports.Just be sure to configure SSD correctly (if any configurations are needed of course).
Otherwise a simple HDD on a simple let's say USB port 2.0 will do just great for you.
I for example ALWAYS use HDD for absolutely everything and NEVER EVER use SSD for absolutely nothing...,but that is just because I have my own damn reasons lol
Cheers to...hmm...to who ever is reading this post lol!