Is it safe to remove USB media when a computer is suspended?

While there are indeed no transfers going on, the reason you do the Safely Remove Hardware dance is not because of the transfers.

You are doing this to cleanly unmount the file system — programs may be still using the drive, and some files may be still unwritten to the disk even after the application using them is closed. This is the same reason why you don’t just hibernate and switch operating systems — a mounted filesystem is never guaranteed to be in a consistent state.

Ever wondered why the OS slows down to a crawl when using floppies? It is because the cache is flushed after the write of each sector, so the floppy can be removed at any time when not in use.

(Still, you will get a dialog and your program will hang if you try to access a floppy that got removed behind your back. Just insert the floppy back and be done. This won’t work with USB drives because they get a different identifier each time — the program will just crash, and the OS may too if a driver was accessing a file)