Not really. I haven't come across any way to force remove disks like this. Since you weren't more specific about what you need to do with your disks, I will try and help you my best:
First off, XenServer 5.6 sp2 allows for each VM to have up to 16 virtual drives! This is pretty cool and it may fix your issues.
I'm assuming you want to start and stop drives like this so you can swap drives among VM's while they are running. I suggest that you make "file server" VM that has an entire drive shared as a network resource using windows file sharing. Then network map this drive on your original windows systems. This will allow you to swap, disconnect, and reconnect drives as you please.
If you "safely remove hardware" for one of your drives, and you want to attach it to another VM without shutting anything down, you can do this.
- safely remove hardware on the drive
- make a snapshot of the vm
- Right click on the snapshot and click "New VM from Snapshot"
- in this new vm, detach the disk you want
- attach it to wherever you want
- when you finally shutdown the original machine, delete the VM.
If you can tell me more about what you want to do by stopping and starting your hardware again, maybe I can give you some better information!