You can buy Teensy from European distributors, for quicker (and hopefully cheaper) shipping.
Regarding USB, for background information to speak meaningfully, you really must read chapter 4 and part of chapter 5 of the USB specification. Many important concepts are explained. Here is the direct link:
This enormous zip file contains 35 documents. The one you want to "usb_20.pdf". There's no need to read all 650 pages! Skip chapters 1-3 (they're useless fluff). Chapter 4 is short, and chapter 5 gets into more detail. If you read only that small portion, you'll learn quite a lot about USB, and you'll become familiar with the USB specific lingo that's necessary to have a meaningful conversation about USB implementations.
Once you have a basic understand of USB device/host roles, endpoints, transfer types and other key concepts, you're ready to read the USB mass storage spec. There are many, but this is the one that matters:
Of course, this only explains how the commands are transported. The actual commands are a subset of SCSI, which sadly is not a free download. There are draft copies floating around, and Jan's USB Complete book(s) contain basically rephrased copies of the specs, at a far lower price than paying for a legal copy of the SCSI specs.
If you read all that, you should have a pretty good idea of the technical issues involved.
Regarding bit-banged USB implementations, they're only 1.5 Mbit/sec. USB flash drives connect their pullup resistor to the D+ line, so communication must begin at 12 Mbit/sec. Of course the host can switch to 480 Mbit/sec after reading the alternate settings descriptor. But the host can not choose to communicate at 1.5 Mbit/sec to such a device.