Moved the AHA-1520C from my 486 VLB build into my P233MMX@430HX build. After some fiddling with it's settings it started working like a charm (I had to set the BIOS at D800 otherwise it would scan for SCSI devices too long 😳 ). SyQuest is visible under Windows 98 with just the drivers above. There are some cool features in this software. I also had to reinstall the Windows because it became unstable when I connected the HDD which I originally got with the case. Turned out it had some vintage virus in the MBR. It was German machine, so perhaps it was a "bubu, bubu.exe" ;D

(No, in reality it was "parity boot" virus - it messed up the Win98 making it unstable for some odd reason, I got rid of it under Windows 10 - Defender had hard time removing it from MBR, but diskpart / clean all succeeded).

So, total success - guy from a software recovery company visited me with his client's SyQuest "diskettes" and we backed them up to USB stick. Although the pentium machine has some strange problems with CRC errors while unzipping files (or even installing software which has to be decompressed from cabs) which were not present before, but it didn't affected the process. Curious thing is that one of the diskettes wasn't readable under Windows with Syquest utils installed, but when I added ASPI4DOS and ASPIDISK in the config.sys and booted windows - it showed the data as second partition (it was 105MB diskette and it showed two partitions - one unformatted 105MB and second - formatted 105MB with all the data, and earlier only the first one was visible).
Funny thing pros have to look for vintage computer geeks to do their work in cases like this 🤣