is tha fact, that my blockchaindata is on an external USB 2.0 drive.
As someone who has been maintaining the block chain constantly for 7 years now on multiple machines, I would personally suggest anyone dealing with block chain data use a SATA SSD at a minimum. Anything less is just asking for additional difficulty. It can work... sure, but don't expect it to be a smooth, or fast, experience.
The faster the storage, the better Core and Armory will work... from my layman's experience.