As workaround in my case I found that I can mount USB flash drive partition as /cdrom and setup will continue:
1. When installer report "Try mounting the CD-ROM?" select "<No>", then "<Continue>".
2. Then in "[?] Ubuntu installer main menu" select "Execute shell", then "<Continue>".
3. Mount flash drive partition as /cdrom, in my case it was /dev/sda1:
mount /dev/sda1 /cdrom
4. Exit shell by entering
exit
5. Select "Detect and mount CD-ROM" in appeared "[?] Ubuntu installer main menu" --- installation will continue.
As workaround in my case I found that I can mount USB flash drive partition as /cdrom and setup will continue:
1. When installer report "Try mounting the CD-ROM?" select "<No>", then "<Continue>".
2. Then in "[?] Ubuntu installer main menu" select "Execute shell", then "<Continue>".
3. Mount flash drive partition as /cdrom, in my case it was /dev/sda1:
mount /dev/sda1 /cdrom
4. Exit shell by entering
exit
5. Select "Detect and mount CD-ROM" in appeared "[?] Ubuntu installer main menu" --- installation will continue.