Well, even on newer Windows versions like 10 or 8, drives A: and B: are specifically reserved for floppies. That is why the actual Windows drive starts on C: and not A: or B:. Just assign it a different drive letter like D: or something that isn't used and it should work. That is probably what @Ramhound meant by an "actual drive letter": something that isn't A: or B:.
EDIT: This may or may not be causing a problem: @Ramhound also mentioned that what you added was a drive label, not a drive letter; that was a mistake on my part. You need to assign it a drive letter still; right click on the partition and click on "Change Drive Letter and Paths;" then assign it a drive letter and click OK. It still is preferable that you assign it something other than A: or B:.