China DOS Union

I used UNDISK to extract the DOS 3.31 image file to drive C: (32M),
rebooted, and it wouldn't run, giving the prompt (invalid system disk).

DOS 3.31 supports hard disks, but you're using it the wrong way.
You should use HD-COPY to read it in and write it to a floppy disk, then you can boot from the floppy.
After booting, run format c: /s, then you can boot from the hard disk. DOS 3.31 supports “large” hard disks, seems like 512M or 2G.

For DOS booting, besides the system files, there also needs to be a piece of boot code in the first sector of the disk. This code is generated by FORMAT /S or SYS. If there are multiple partitions on the hard disk, you also have to set an active partition that can boot.

Thanks.
DOS 3.31 is a very successful system made by MS, much more successful than DOS 4.0.

DOS 3.31 seems to support only 32M, right? To support large hard disks it seems SHARE command has to be used, I haven't tried it, getting ready to try.

So DOS 3.31 can handle a 512M hard disk. At first I thought it could only handle 32M, then 2G definitely wouldn't work.

format c:/s transferred the COMMAND.COM command interpreter to drive C:.
If I copy all the files from drive A to C, can I do without drive A, and those hidden files,
do I not need to copy them?

After format c:/s, you can boot DOS from the hard disk, and the remaining files on the floppy can be copied to the hard disk for use.
But note that any related paths pointing to drive A: in CONFIG.SYS and autoexec.bat need to be changed.

Thank you, Teacher Mo

The DOS versions that can only use a 32M hard disk are DOS 3.30 and earlier.