Hello,
When I copy an unencrypted file from a USB drive to a vault on another USB drive, the files first get copied to my C drive. Is there a way to change the “working” drive from C to another drive of my choice?
Thank you!
Welcome to the Cryptomator Community 
from your description i would say this has nothing to do with Cryptomator.
I mean, if your vault is mounted into a folder somewhere on the C:-Volume, of course it looks like the C: is used as a temporary storage, but in fact it is not. Cryptomator en- and decrypts on the fly, i.e. on your real storage device only encrypted data is present and your unlocked vault is just a decrypted view stored in memory.
How do you know that the file is copied first onto your c drive and only in a second step copied to the final destination?
Nothing scientific, but I use Total Commander and repeatedly use CTRL-R (for refresh) on my C drive and as the files get copied, I see the size of my hard drive go down then back up for the size of the files I’m copying. That leads me to think that the C drive is used somehow. My C drive is an SSD.
Please look at my topic to see a more detailed view of the problem.