So I have some photos I took from my camera's SD card stored onto a spare USB drive that I have (cause my laptop doesn't have a SD card reader so I just use a roommate's laptop that does and transfer it to my USB drive), and I go to eject the drive after looking at the pictures, but its says its unable to because its still in use. The only thing on that drive are photos, but nothing is open. What do I do?
It sounds like one of two things is happening: either some process still has a photo file open, or the transfer photos to the USB isn't complete (i.e., file copy still taking place, or the filesystem isn't finished the write or write verification, depending on your OS or filesystem).
In the most inconvenient case, simply shutting down the laptop (safely) will allow you to remove the USB drive. If the copy isn't complete, the shutdown might be delayed until the copy is finished. But regardless, if the machine shuts down without errors, you can remove the USB drive, and know that the files are probably correctly written.
If the machine doesn't shutdown normally, or requires a force power off, then you should be concerned about the integrity of the files on the USB drive. Use whatever methods you are familiar with to verify the files are there, and that they contain what you expect them to. The exact method is beyond the scope of this answer (because it depends on your backup & verification practices that existed before this occurred).
What OS? If you want to be certain of doing the right thing, shut the computer down and then remove the USB drive. If you're using Windows, I suggest freeware LockHunter application that can show you what application has locked your files.
Also, do not keep only a single copy of your photos. All drives fail/die - magnetic and flash-based. Don't wait until it's too late. Back-up now.
And... buy a card reader. E.g. https://www.amazon.co.uk/gp/product/B07HB866XN/
It can be an open file or an open directory (file explorer window....). In any case, after a couple of minutes the filesystem on the SD card should be in sync and there should be no damage if you remove the card if all files have been transferred.
This said, a USB SD card reader costs a few dollars/euros, and can be much faster than a built-in one: on my PC the integrated SD card reader uses USB2.0 electronics while an external one (basic model from Amazon) in the right port uses USB3.0.