I've got a W520 running Windows 7, and recently I got an off-brand Ultrabay caddy off eBay so I could install an internal HDD in the optical drive slot. I get to the point a bit further down so feel free to skip ahead to the bold text.
(For a bit of context, I was given this laptop by my dad - he's the real ThinkPad collector - so it was mostly upgraded by him until now. Pretty sure the boot drive is an SSD, and with his old stuff and all my digital-hoard it's a bit full. Slowly gearing up to getting an external HDD to back things up so I can clean it up a bit and let the SSD breathe, but in the mean time I have managed to make a decent little bit of space. However, definitely hasn't got enough space free for all the games I'm installing from my Steam account now that I have a USB controller to play them with. These are games I've been itching to play for about a decade and finally, thanks to my dad's generosity, I have a laptop that can handle them. I was hesitant about ThinkPad's at first; I cannot stand the little red dot

Right, back to the point. I've managed to install the caddy and the HDD all fine - it took me a bit to figure out that I needed to format the drive for Windows. I'm very used to Macbook's OS and external drives working out of the box. Ended up finding an article on ghacks by Martin Brinkmann (forever grateful) explaining clearly how to do so.
I was content that it now recognised the drive for what it was, and considered the problem solved.
Now, to the issue I'm having:
When I put the ThinkPad to sleep overnight (I know, bad habit from Macbook times) or when it goes idle by itself after a period of inactivity, on waking I get an error.
This doesn't seem quite right to me... I intended to have this HDD installed as a storage drive, and I want it to behave like any other internal HDD. It seems like the system is treating it as removable storage, like a flash drive/USB, instead. I'm not sure what's causing the issue and have searched around without much luck. (Some Win7 forums suggest an AHCI issue, others ESATA? I have no idea and I'm a bit overwhelmed by the possibilities at this point.) So, I'm not sure what to ask and am open to any ideas (or if you had the same issue and know how to fix it, or can see what I've done wrong setting it up) but I wonder if it's ignorance on my part about how the Ultrabay optical drive HDD caddy connects up?Problem ejecting <internal HDD - Toshiba 1TB SATA iii>
Windows can't stop your generic volume device because a program is still using it close any programs that might be using the device and then try again later
The drive was sold as SATA III, as was the caddy, but I'm oblivious to how the optical drive in a ThinkPad/W520 connects or if I'm missing something about how to set up the Ultrabay.
There are so many moving parts here (my HDD formatting setup, the various hardware, Windows7, etc.) I am just a bit lost for what to check first.
I've done a lot of searching in here and elsewhere already and not found a solution which leads me to believe I've missed something obvious. (I know there's things like hotswitching or whatever it's called, and the option in the drive properties regarding "removal policy", but that that shows up at all strikes me as being part of the problem. And I don't want a workaround that just makes "Safely Remove Hardware" stop showing up.)
So, that's a lot of words - apologies. TL;DR - why is my 1TB SATA III internal HDD in my off-brand Ultrabay optical drive caddy seemingly recognised by Windows as removable (flash/USB) storage, and how can I fix it so that it's treated as a permanent connection to be put to sleep in the same way as the boot drive?
Thanks for reading and any suggestions!