Not every brand and model of USB memory key is compatible with the V-mixer. 

Most, but not all, major brand units should work. Many cheap, no name brands do not work due to speed, spec or quality of component issues. We can verify that the following brands/models will work.

Kingston Data Traveler 100 DT100/1GBKR 1GB 
Kingston Data Traveler DTI / 8GB 8GB 
Kingston Data Traveler 100 DT100/16GB 16GB 
Kingston Data Traveler 150 DT150/32GB 32GB 
SONY USB Flash Drive USM1GJ 1GB 
PNY OPTIMA PRO attache P-FD2GBHSP-F 2GB 
PNY attache P-FD01GU20-RF 1GB 
Lexar JumpDrive Lightning JDP1GB 1GB 
Lexar JumpDrive FireFly JPFF512 512MB 
imation Clip Flash Drive Flash-Clip1G 1GB 
Ativa Portable Flash Drive JDON1GB 1GB 
Memorex Travel drive 32509067 1GB 
Memorex USB Flash Drive 32535064 64MB 
(as this test data is older, larger size drives exist now and most but not all larger sizes should work as well)

Unfortunately because there are thousands of flash models available and from hundreds of manufacturers or OEMs, it makes it impossible to test thoroughly.
We have had varying levels of success when testing SanDisk units which tend to be sporadic from model to model. If you are looking at purchasing a memory key, we recommend Kingston Data Traveler. 

Please note the following: 

• Be sure that the V-Mixer is operating at it's latest version of firmware:
https://proav.roland.com/global/products/m-200i/downloads/

• Just because your memory key will work in a PC or Mac it does not guarantee it will work in the V-Mixer. The PC and Mac have a large general purpose operating system with multiple drivers and the V-Mixer does not. It is a USB host chip that supports USB flash keys only.

• Ensure that your memory key is truly empty. A number of companies load hidden applications in these keys that automatically try to boot up when you insert them in your PC/Mac. When you look on the drive, you cannot see these and it actually looks like the unit is empty. These types of memory keys will not work in the V-Mixer. We have had varying success when we uninstall the application but finding a way to do so can involve quite a search. Some manufacturers have an uninstall tool on their website. Here are the links on SanDisk's Support FAQ's that explain how: On a PC:
http://kb.sandisk.com/app/answers/detail/a_id/2550/kw/cruzer%20initialize/r_id/101834 On a Mac: http://kb.sandisk.com/app/answers/detail/a_id/826/kw/cruzer%20remove%20u3%20mac/r_id/101834 

• Ensure your USB memory will pass the Speed Test - in the System>USB Memory Menu.

• Your memory key must be formatted in the V-Mixer. Formatting it in your PC/Mac will not suffice:
https://www.rolandus.com/proav/knowledge_base/218363343

• If you find that after trying any of the above, your key continues to fail, then it is highly likely that it is just not compatible or is not functioning properly. In this case, test with a different USB drive from a name brand.