I've recently encountered a situation when reattaching a forgotten (or otherwise unknown) LVMoHBA or LVMoISCSI SR where you are presented with a dialog which tells you that a previous SR has been found and that you need to select either to "Reattach" or to "Format" the device in order to proceed.
Here's the rub (and the problem) - the buttons you need to press aren't present.
This has occurred to me using XenCenter 5.5.2, on Windows 7. I expect it could also occur on Windows 2008, particularly on the R2 version. It might occur with other revs of XenCenter.
Here's the work around - Run XenCenter 5.5.2 (or whatever is right for your environment) on Windows XP or Server 2003. The dialog will display properly and you will be able to complete the reattach process.
