General info, hardware and software wise:)  Recommended to be on Hakchi CE, most current version...to best utilize USB-HOST!

OTG Device Wise:

Slow: Standard Set-Up would be, as an example, a Kirin Octopus shaped OTG with a Sandisk 32 GB Flash Drive.  The Octopus OTG devices are
cheaply made, in general.  So, do not expect them to be durable!  But, they are OK for usage if you have no other easy alternative.
You can run a flash drive or hard drive, along with 2 peripherals, such as a mouse and keyboard.  

Medium: Better and more reliable set-Up would be the Left-Angle Device, which is quite durable.  But, you can only run Flash Drive or 
Hard Drive.

Fast: The best Set-Up is the Inatech, or similar HUB styled OTG Device.  Speeds are fast.  Device is durable and reliable.  Plus, you
can have SD card as well as a few devices!

USB Device Wise:

Slow: Flash Drive: For the most part, they work just fine!

Medium: Passport HDD: Very good speeds.  No complaints!

Fast: Passport SSD: Excellent speeds!

Format Wise:

Slow: Fat32

Medium: NTFS

Fast: EXT4

Game Number Limit:

25 Games per directory or folder, is the safe and magic number!  You will be far less likely to run into any sort of C8 Errors or 
Save State issues, this way!

Sincerely, KMFDManic!