This has been written about a hundred times or so, but I'm writing it here so it can be found all in one spot (for me as well as my readers). I had to go all over the web to gather bits and pieces of this information.
My Veeam server (running Windows 2008 R2) has a fiber connection (though this same principal applies to iSCSI). One of the scariest things I have ever done in my nine years of IT has been to let this Windows server "see" my production VMFS volumes. When I say "see" what I mean is that in the Windows disk management console, your VMFS volumes will be listed, but will be offline and not mounted in Windows with a drive letter.
BEFORE you give your server access to your VMFS volumes on the SAN side through an access control list of some sort, you need to prep your Windows server first. I have read that Veeam does this automatically on installation, but I'd rather do it myself and know absolutely that it's done. From everything I've read, if Windows initializes a VMFS volume and assigns it a drive letter, then that VMFS volume is hosed (shudder). Even worse, you're probably just installing Veeam, so you don't even have the best virtual machine backup software protecting you yet!
On your Windows box, you should do the following:
1. Open a command prompt (run as administrator)
2. Type this command and press enter: diskpart
This opens the diskpart shell
3. Type this command and press enter: san
This returns the current SAN policy. The SAN policy is what tells the system to automatically bring new media online or offline. What you want to see is "Offline All". If you see "Online All", simply enter the following to change it: san Policy=OfflineAll
Type the SAN command again to ensure it's set the way you need it.
4. Type this command and press enter: automount
This returns the current automount policy. You want to see the automatic mounting of new volumes DISABLED!!!!!! If yours returns enabled, run the following command: automount disable
5. Reboot your system so the changes take effect.
After rebooting, it pays to check the policies again. Half a minute now could save you a lot of explaining and hours of restoring from backups later. If you need help from within diskpart, or just want to read more about the syntax, type help within the shell. It also works for commands, like so: help san.