After getting annoyed at the failure rate of my old 3.5' floppy disks I started using a Gotek USB Floppy emulator on my Amiga and Atari ST. Being impressed with how well that worked I decided to give it a shot on my trusty ZX Spectrum +3 who's disks are as equally unreliable, even harder to source and also very expensive.
On my other Gotek drives I use the FlashFloppy firmware (still in active development at time of writing v0.9.27a - Aug'18) and recent developments have included .DSK support for the Amstrad CPC & Spectrum +3 (basically the same format). Therefore as a test I mocked up some connectors (as described here), plugged it all in and got it running nicely with compatibility at 100% of the disks I've tested. A list of the TOSEC collection tested disks can be found here. I've also written a small utility which removes the need to re-dump the weak sector protected disks by adding this extra information to the eDSK automatically. You can download a Windows executable here or follow the instructions detailed on the FlashFloppy wiki here.
I was impressed enough to decide to permanently replace the internal drives of a couple of my machines, including creating a 3D printed drive caddy as the Gotek case won't fit in a 3' drive bay. Also as there is no file-selector for the ZX Spectrum I added a rotary encoder with OLED display to navigate the files on the USB stick, and topped it off with a sound buzzer to emulate the sound of the original drive.
Below is a guide on how I built all of this, heavily utilising the resources of the web, including the parts I used and links to the 3D printed caddy I designed so you can print your own.
A lot of the information below is from the FlashFloppy wiki so I highly recommend you read that to get more information and more detailed schematics.

• Amiga Hardware Database - Elaborate Bytes / BSC A.L.F. ST506/412 or SCSI controller; A.L.F. 2 is a general hard disk driver software with 16 different hardware versions based on it.
• ・Autoboot mode Amigaなど主に海外レトロPC向けなので、ここでは説明省略. 0．ジャンパの設定 GOTEKが1台の場合、ジャンパピンを”S1”から“S0”に変更 GOTEKが2台の場合、1台はジャンパピンを”S1”から“S0”に変更 1．設定ファイル.
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Floppy sound mod kit for. Gotek usb floppy emulator. Sound mod only!! - gotek not included. No soldering - connects direct to gotek jumper pins. Works for both ff and hxc. firmware.

Amiga Gotek F Facebook

The most unreliable part of the MSX is probably the floppy drive. The drives are getting old and unreliable.
They can be replaced with pc drives but a nice alternative is a Gotek drive.

The Gotek drive emulates an floppy drive. It connects to the floppy cable just like a regular pc floppy drive. So if the MSX is able or converted to use pc floppy drives, it can also use the Gotek drive. Instead of floppies it uses a regular USB flash drive.

By default the Gotek comes with 7 segment display on which you can see the number of the image selected with the buttons on the Gotek.
It is possible to replace this with a lcd or oled screen so you can see the filename of the image selected.

It is also possible to connect a speaker to emulate the stepping motor sound for those who miss it.

The standard firmware on the Gotek is not very useful, but luckily there are alternatives. First there is the HXC firmware.
This costs 10 euro and allows the Gotek to read HXE images from the USB flash drive. DSK and DMK images can be converted to HXE with the included tool.

Alternatively there is the open source FlashFloppy firmware. With the FlashFloppy firmware it is possible 360k and 720k dsk images directly from the USB flash drive. It can also use HXE images so it's still possible to convert DMK images to HXE and use those.

F&f.hu

Because it connects to the floppy controller in the MSX is also emulates the actual speed of the floppy drive. This means it's pretty slow compared to using floppy images with a mass storage device like the MFR of Carnivore.
However it is a lot cheaper and it also supports most (perhaps all...?) copy protections.

The Gotek drive can be bought quite cheap from places like Aliexpress.

Amiga Gotek F Font

Relevant link: HXC firmware
Relevant link: Updated FlashFloppy firmware

Thanks to Keir Fraser for the firmware (Flash Floppy)