Can two USB flash drives conflict with each other?

[bob prohaska]

I just tried connecting a second SanDisk USB 3.0 flash drive to
a Pi3 running -current. It's not precisely identical to the first,
having a different activity indicator and being slightly older.

Upon connection, the RPi3 console began emitting a continuous loop
of error messages:

(da0:umass-sim0:0:0:0): Error 5, Retries exhausted
g_vfs_done():da0a[WRITE(offset=268632064, length=32768)]error = 5
g_vfs_done():da0a[WRITE(offset=825819136, length=32768)]error = 5
swap_pager: I/O error - pagein failed; blkno 2228,size 4096, error 5
gv_vfsm_done_f()aul:dt:a0 pa[aWger RIreTE(ofadfs eetr=ror,65 p536id,  1leng t(iniht)
=4096)]error = 5
g_vfs_done():da0a[WRITE(offset=8585216, length=4096)]error = 5
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 00 08 01 80 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 3 more tries remain

This output seems to go on indefinitely, even after the offending
flash drive was removed. Note the garbling in the 5th line of console
output; it's been seen on rare occasions before, but never "on demand".
A shutdown -r command stopped at

Shutdown NOW!
shutdown: [pid 3469]
root at www:/usr/src # sh: /usr/bin/wall: Input/output error

and the shell is now unresponsive. The console still has a login prompt,
but never asks for a password. Looks like plug-pulling time.

The obvious solution is "don't do that!", but if somebody can offer
a more insightful explanation I'd be grateful. Using two USB flash
drives simultaneously would be very useful.

Thanks for reading,

bob prohaska