B+ with external USB drive won't boot with Jessie reinstall

[MacsandaPi]

I've been running a BOINC project on my Pi B+ (Jessie) from an external hard drive, and recently upgraded to Stretch. I then discovered that the BOINC application is incompatible with Stretch, so I attempted to return to Jessie, but now the boot process fails with the external drive: the light on the hard drive flashes on briefly when the Pi is powered (the external drive has a separate power supply), but then nothing happens.

When I first set this up successfully about a year ago, I altered the default boot path on the SD card by editing the cmdline.txt to change

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root=/dev/mmcblk0p2

to

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root=/dev/sda2

. Now, after downloading the 2016-09-23 version of Jessie, this change has no effect. I've also tried adding

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rootdelay=5

and

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program_usb_timeout=1

as noted in the RPI Advanced Users Forum "Sticky:How to Move the filesystem to a USB stick/Drive." These additions made no difference.

The B+ with external drive worked fine previously, but I don't know how to overcome the problems mentioned above. I assume the problem is related to the cmdline.txt file, but I don't know what else to do. Thanks for any help.

[asandford]

I run a Pi with an external (powered) hdd, here's me cmdline.txt:

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root@sandford:~# cat /boot/cmdline.txt
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 console=tty1 root=/dev/sda1 rootfstype=ext4 elevator=deadline rootwait
root@sandford:~#

Make sure the /etc/fstab also references the correct partitions:

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root@sandford:~# cat /etc/fstab
proc            /proc           proc    defaults          0       0
/dev/mmcblk0p1  /boot           vfat    defaults          0       2
/dev/sda1  /               ext4    defaults,noatime  0       1
# a swapfile is not a swap partition, no line here
#   use  dphys-swapfile swap[on|off]  for that
root@sandford:~#