HP Color LaserJet Enterprise CM4540 MFP series - Manual (Page 335)

The file that you want to print is not listed in the Open from USB
menu
1. You might be trying to print a file type that the USB printing feature does not support. The product
supports .pdf, .prn, .pcl, .ps, and .cht file types.
2. You might have too many files in a single folder on the USB storage accessory. Reduce the
number of files in the folder by moving them to subfolders.
3. You might be using a character set for the file name that the product does not support. In this case,
the product replaces the file names with characters from a different character set. Rename the files
using ASCII characters.
ENWW Solve walk-up USB printing problems 315