I have retraced my steps using Windows 10 and vmworkstation 9.0.0 build 812388
1. Insert a 1gb usb stick formatted to fat32 before opening vmware
2. open vmware, grant admin permission or open as administrator
3. File - > new virtual machine
4. Typical - > next
5. option 2 (installer disc image iso) - > browse and chose dos6.iso - > next
6. os - > other version - > ms-dos - > next
7. chose name - > next
8. split virtual disks - > next
9. finish
10. chose the newly created vm - > under devices clink hard drive = new window opens with "virtual machine settings"
11. delete the current default hard disk (2gb default)
12. add - > hard disk - > "use a physical disk" - > next
13. now under device pull down you will get a list of physical drives, I have 1 main hard disk and 1 usb, not sure how your system is. so for my system Physicaldrive0 = hard disk Physicaldrive1 = 1gb usb drive
in my example I select the 1gb drive (physical drive1) - > use entire disc - > next
14. click finish then ok
15. power on vm
16 dos6 boots to A: drive now c: is your usb drive
obviously once the usb drive is mounted on vm it is not accessible through windows 10 so copy all your files there beforehand
I did have a error saying it wouldn't mount at one stage but rebooting the system with the usb in the usb port and straying vmware fixed it.
I have used this method a few times to fdisk, format and make cf cards active drives for my retro pcs running dos and always works very well. The great thing about this method is the usb disk is a primary drive as the iso is booted through a cdrom. If you attack a usb/cf card as a secondary drive Fdisk will not allow to make it active as it sees it as a secondary not a primary drive
sorry I did remember it wrong, I was using a usb drive all along not mounting a folder from a local hard disk