“Cannot access network drive”

I haven't seen the problem you're seeing, but I have "shared" memory card readers, external USB-attached disk drives, and external firewire-attached disk drives with no ill effects in Windows Server 2003 and Windows XP. I've never used any of these Drobo devices, but if they're just a USB Mass Storage profile they should work fine.

It may be that the "Server Service" isn't "seeing" the drive. Try doing a "Restart" on that service after the drive is attached and working, and see if that improves things for you.

Adding the drive as an NTFS mount point might just change the behaviour, but I can't say for sure. I'm at a bit of a loss at this moment as to tell you why you're not able to add it as a mount point, though. (Just for kicks, I just added a FAT-formatted SD memory card in a USB reader to my Windows XP machine here with no ill effects.)

IIS's default operational mode is quite locked down. IIS doesn't use <computername>\NETWORK SERVICE. It uses <computername>\IUSR_<computername>, which is a member of the <computername>\Guests group. If the web page you're trying to access on the IIS server instead is requesting authentication, it will instead attempt to use those credentials to authenticate. If the files it's accessing are executable scripts, then instead it uses IWAM_<computername>. I don't remember offhand the group membership of that.

What you want to do here isn't very easy to configure over a workgroup. You'd have to set up an account on the remote machine with the same username and password that the IIS system is using. You can use a script to hopefully determine the IIS user account's password, but I don't know if IIS changes that periodically or if the script doesn't function on newer versions.