When logging on to Windows 7 user gets a second mapped drive to the users home folder

Question

Morning,
I'm working through an issue we've discovered whilst trialing Windows 7.  Our environment is setup as follows.
Domain Controllers are Windows Server 2003
Clients are Windows XP and Windows 7
Windows XP and Windows 7 Clients are in seperate OU's
All Windows XP Group Policies apply to Windows XP And Windows 7 clients, Windows 7 policies are then applied to Windows 7 clients after
Windows 7 policies are setup such that any setting defined in a Windows XP Group policy is left unconfigured in the Windows 7 Group policy and only new Windows 7 settings have been set in Windows 7 Group Policies.
We have users home folders mapped in their Account Directory account setting and set to H:\   to connect to
\\domainname\dfs\home\username
In addition we also as a fail safe map the drive via a login script using net use
This has worked fine for years in windows XP, if Active Directory failed to map the drive for any reason then the login script would then map the home drive.
In Windows 7 we have noticed a curious error.  We found after a period of a couple of weeks we suddenly started getting a new drive mapped.  This was identical to the H:\ drive mapping but was instead under drive Z:\.  In other words, the
users home folder is mapped twice on h:\ and z:\.  This is not affecting any of the Windows XP users.
I have gone through several logic reasons to ascertain why this has happened with the following findings.
1. Originally we thought the error appeared when we tried out mapping the home drive using the mapped drives functionality new in Windows 7 group policy under preferences > windows settings > drive maps.  However, after forcing it to delete the
Z:\ drive using this functionality we only succeeded in removing it with a group policy present to do it.  As soon as we removed that group policy the Z:\ drive came back
2. Secondly i thought the reason we would be getting a z:\ drive when we haven't specified it anywhere is because active directory is trying to map to the H:\ Drive but it is already present therefore in Windows 7 it tries to map to a different drive. 
Using Windows logic it tries the highest letter first which is unlikely to be in use i.e. z:\.  This makes sense because our logon script uses a net use h:\ command to map the drive and i believe by default these are set to perisistent.  Therefore
the next time the user logs on H:\ is already mapped so the logic in Active Directory accounts maps the drive to Z:\ instead hence we end up with two mapped home drives.  To test this i altered the login script to set the drive maps to
non persistent using persistent:no.  The logic here was that when the user logged off the drive would become unmapped so that when Active Directory tried to map the drive it would be able to use H:\.  Unfortunately this was not the case and
Z:\ remains.
3. Here is where i resolve the issue but i don't know why and is the bit i need answering.  If i go into my account on Active Directory and go to Profile and set the home folder drive letter to another letter i.e. change from H:\ to U:\,
i get prompted to set full control etc and apply.  I then set the drive back from U:\ to H:\, again i am prompted for setting full control which i accept.  
NOW when i login i no longer receive a Z:\ drive and only get an H:\ drive.  YAY, thats what i want, however, i do not understand why this is the case.  At first i thought it might be something in the active directory logic when i login to an XP
machine and then login to a windows 7 machine.  IF you bear in mind that although i get a new profile in Windows 7 i still retain the same home folder setting.  However, after logging on an XP machine logging off then logging
on a Windows 7 machine i still didn't get the Z:\ drive back. 
I have tried creating a new user that ONLY receives our Windows 7 Group Policies and still they receive both an H:\ and a Z:\ drive which rules out the Windows XP policies conflicting with the Windows 7 policies.
Does anyone have any ideas why i would get a second drive mapped to Z:\ logging into Windows 7 on a Windows Server 2003 domain? 
Can anyone explain in more detail exactly how the Active Directory functionality works when you specify a connect to Drive letter for a users home folder?
My current workaround is simply to add a net use command to remove the Z:\ drive.  I do not want to use Mapped Network drives using the new policy settings in Windows 7 RSAT because we have already found issues with it. 
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Answer 1

We had the exact same issue happen to us. No issue on Windows XP but Windows 7 mapped two home drives (F and Z). F was mapped during the login script and Z was being mapped by some unknown reason.
We use Netapp for our storage and home drives and use a feature called CIFS Home Drive Mapping. This essentially maps a user to a folder not available through normal CIFS methods causing AD to error when setting the home drive. To get around this we use
dsquery and dsmod to modify this attribute in the user account. This is the command we used...
dsquery user -name %UserNameX% | dsmod user -hmdrv F -hmdir \\filer\$username$
Notice there is no ":" after the F
We determined the root causewas the value for the
homeDrive attributefor the user account was set to "F" instead of "F:"
When we updated our users to reference "F:" instead of "F" in their user, this problem was resolved.
Use LDP or some other method to verify that the homeDrive attribute is set to "F:" (or any other letter) or you will have this issue.