I'm using the 1Password4 beta, and I also have a Filevault-encrypted USB flash drive. The only thing on the drive is a GPG keychain and a Mac .keychain file (from Keychain Access). It's not the System or Login keychain, it's a fully separate one that I've created and placed there.
About 90% of the time when I try to unmount the flash drive, OSX refuses to do so, telling me that 1Password4 is preventing the drive from unmounting. If I quite 1Password4 (both the main app and the mini bar thing), unmounting stalls with a popup that the drive is still being accessed, and I can choose to forcibly unmount it.