This pertains to an ISCSI target setup on a nas4free VM on ESXi 5.1.0
nas4free 9.2.0.1 - Shigawire (revision 972)
FreeBSD 9.2-RELEASE-p4 (kern.osreldate: 902001).
The LUNs were created on a USB hardisk permanently attached to the VM though the VM settings controls.

I created two targets. I was mounting one in a windows system that may have been Windows XP and the other I was mounting in a Debian 8 system. The Nas4free VM became inoperable. The Windows XP box has also died. So I moved the USB hard drive to another nas4free VM that had the same setup.

I am now attempting to mount the ISCSI target with a pretty up to date Windows 10 system but the disk shows up as a raw disk. Is this normal? If so, why does it behave that way?

How did you add the disk to the new NAS and tell it to not clear the disk?

If I understand your question correctly . Windows will not read the usb disk because it is (probably) formatted as some Linux file system (ext 2,3,4 or other, hopefully the data not on MD or VFS volumes ). So yes that is normal for windows. Even after all these years there is no support for Linux file systems under windows. There are some tools and drivers out there that will allow windows to use Linux filesystems. A quick search came up with Paragon having one and some others. But I have no clue if they will work with your disk as a ISCSI target disk. With windows generally speaking if windows can read/write the disk it can be used as a target but I have only ever used ntfs disks as targets.

Your best bet might be to get a raspberry or some such and install Debian on it and set that up as a ISCSI target box if it is for casual use or use an old junk pc you may have laying around .

In the future if you need to have a target disk that may be used on windows you will need to format it as ntfs or efat and use it that way ( but you will take a performance hit, Linux is pretty slow with ntfs ).

.....and then again the easiest thing would be to copy all your data over to a ntfs or efat disk and use that as a target then format your original disk for use on windows or keep it as a backup or even copy your data back onto it.You would just need a linux boot disk or system on a usb stick.

To answer your other question why it acts that way. Remember Iscsi is just a software abstraction on top of the actual hardware that is used to store the data the actual data is on whatever storage medium you choose to use as a target hardrives,cdroms,ramdisks..etc.

...or if you meant why windows see that disk as a raw disk it is because windows does not know how to read it.

I decided not to pursue this question any longer since I might be a little fuzzy on the details of what I did after the initial failure, which by the way actually happened quite sometime ago. I did assign the USB drive to a different virtual machine running Nas4free and I am unsure if I could have done something in the process to clear the disk of it's formatting. I formatted it as NTFS when I mounted it as an ISCSI target in the initial Windows system. But after my recovery attempt it did not come back as NTFS. I thought that I would reattach and find the NTFS drive intact with all the files that been stored there.