A couple revisions to my prior post as I kept digging further due to a case of insomnia. The Gotek USB's are now Read/Write compatible. No issues sampling in and saving it to the USB.
1.2 firmware does indeed just load off the system, should load upon boot in a factory stock machine- not booting 1.2 even with an empty disc drive is signs of an error.
Lastly, concerning the OS4 and the ASK trigger, even if it doesn't work with OS4, you can just not have the OS4 in the drive which means it boots 1.2 again and you have full access to the ASK trigger functions again. Just be sure to keep 2 seperate groups of files as 1.2 can't use files/samples created/saved when running OS4 and vice versa.
Think of OS4 as turning your S900 into a S950 (or perhaps more along the line of a "S925"

) only when you want to. It still has the memory and file limitations of the S900 though. It's just tempory at your discretion. Akai's tech specs:
INTRODUCTION
Thank you for purchasing the S9V4.0, an exclusive system disk for the
AKAI S900.
The S9V4.0 has been developed to augment the functions of the S900.
By loading this system software onto your S900 V1.2 (1,2), you can
obtain the following additional functions and upgrade the musical
expressiveness of your S900.
1. Pre-trigger recording
2. Special one shot mode
3. Crossfade looping
4. Dynamic Filtering
5. Automatic locating of start point
6. Velocity release by note on
7. Combine two samples for velocity crossfade type samples
8. Changing the velocity mix curve for velocity crossfade
9. Time skew for velocity
10. Key group copy function
11. Disk mode: Compression of sampled data
When using S9V4.0, please make sure you have fully mastered the basic
operation of S900.
WARNING
* The S9V4.0 cannot be loaded unless your S900 is V1.2 (1,2).
* The S9V4.0 cannot be copied. If you have trouble with your disk
under normal use, please consult the Service Division of AKAI's Head
Office through the store where it was purchased.
* Because of the augmented MIDI exclusive function, you may get an
error message if you load an editing software which was not created
by this system software. Please be sure to load this system software
before using editing software and sample data created by S9V4.0.