Universal Serial Interface for Amstrad CPC (a.k.a. USIfAC II), is a board designed for Amstrad CPC 464/664/6128 offering many amazing features:

• Incredible loading speeds of up to ~30kb/sec (6-7 times faster than a Real Disk Drive)!
• Use a Usb stick or PC Hard Drive (through ESP8266 WiFi module or UART serial cable) to: Change/create/delete directories and load/save/delete programs/games even Ascii files!
• Full read/write access to DSK images of both AMSODS and PARADOS formats!
• Access up to four DSK images and choose them "on the fly", to support all multi-image Games!
• Load .SNA snapshot files!
• Many RSX commands (Basic commands used directly) for: Copy files from/to floppy disks, transfer DSK images to floppy disks, backup floppy disks to dsk image files, quick Format disks, and many more!
• Equipped with a Reset button and a Pause switch

This is probably the Best "value for money" Board for Amstrad CPC 664/6128, but for CPC 464 i suggest ULIfAC board And here are some demonstration vidoeos:

USIfAC II presentation on Amstrad CPC 464

USIfAC II presentation on Amstrad CPC 6128 (in French)

USIfAC II board review on CPC 464