It is technically possible (as in "shooting-yourself-in-the-foot"), but highly unadvisable, especially if any of the disk's partitions are writable.
Picture this scenario:
dd starts reading the disk at the start, and merrily makes its way towards the end.
- When it's halfway there, the OS writes a file to the disk. The file is somewhat fragmented though - one part of it is physically located towards the beginning of the disk, and another towards the end.
- No problem for the OS - it writes the file just fine, and pushes it out of its write caches; it is now correctly written on the disk.
- The problem is with your backup:
dd has already gone past and copied the first part of the file, so it will capture an inconsistent state when it gets to the second part - each part will be from a different version!
If you're capable of remounting all the partitions on the USB disk as read-only, this problem shouldn't occur (emphasis on "should not"); this requires significant up-front install/offline preparation and boot-time setup for the
/ partition though - I don't think you can normally remount
/ r/o on-the-fly, there will be multiple things hanging off that requiring r/w access.
So, running the copy probably won't break the running system, but won't give you a workable backup, rendering the task moot. I highly recommend making the copy offline - which would require shutting down the machine.