Computer fails to sleep

Question

When I put computer to sleep, after approximately a 2 second pause, it wakes up again.

Answer 1

From other suggestions, I've disabled Guest User, with no change; still wakes up, From pmset -g assertions, I got the following:
Listed by owning process:
  pid 29: [0x0000001d012c0000] ExternalMedia named: "com.apple.powermanagement.externalmediamounted"
Kernel Assertions: 0x0004
* Kernel Assertion ID = 500
   Created At = 12/31/69, 4:03:12 PM PST
   Modified At = 12/31/69, 4:09:03 PM PST
   Owner ID = 0xffffff800cbbd000
   Level = 255
   Assertions Set = None (4)
* Kernel Assertion ID = 501
   Created At = 12/31/69, 4:03:22 PM PST
   Modified At = 12/31/69, 4:08:57 PM PST
   Owner ID = 0xffffff800cb3b000
   Level = 255
   Assertions Set = None (4)
* Kernel Assertion ID = 502
   Created At = 12/31/69, 4:04:53 PM PST
   Modified At = 12/31/69, 4:00:00 PM PST
   Owner ID = 0xffffff80847a4000
   Level = 0
   Assertions Set = None (4)
* Kernel Assertion ID = 503
   Created At = 12/31/69, 4:05:01 PM PST
   Modified At = 7/13/12, 5:59:48 AM PDT 
   Owner ID = 0xffffff80847aa000
   Level = 0
   Assertions Set = None (4)
* Kernel Assertion ID = 504
   Created At = 12/31/69, 4:05:19 PM PST
   Modified At = 12/31/69, 4:00:00 PM PST
   Owner ID = 0xffffff80847ad000
   Level = 0
   Assertions Set = None (4)
* Kernel Assertion ID = 505
   Created At = 12/31/69, 4:05:24 PM PST
   Modified At = 12/31/69, 4:00:00 PM PST
   Owner ID = 0xffffff80847a7000
   Level = 0
   Assertions Set = None (4)
* Kernel Assertion ID = 506
   Created At = 12/31/69, 4:05:29 PM PST
   Modified At = 12/31/69, 4:00:00 PM PST
   Owner ID = 0xffffff80847f1000
   Level = 0
   Assertions Set = None (4)
I have a backup disk mounted (along with a Backup.spareseimage icon)  to run in Time Management. Is this the external media mounted referred to above?