Tried to create virtual disk at startup by adding the job in the task scheduler. Manual starting the job works fine, but not at startup - no new disk appeared after rebooting. Digging a bit deeper revealed the problem - there was an error message issued by imdisk exe when it was started at system startup by task scheduler.
Changing startup type for "ImDisk Virtual Disk Driver" from "Automatic" to "System" solved that problem. It works fine now.
Hope this will help to somebody.